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This book attempts to give in broad, schematic form 
the conception of the structure of the material universe 
which has developed in the minds of modern students 
of physical science. The treatment of the subject 
which is here offered is radically elementary, and is 
intended to be "popular" if not "literary" in its 
style. But, although elementary, it omits none of the 
salient general ideas, whether these belong primarily 
to the sciences of chemistry, electricity, optics, or heat. 
It is characteristic of the modern standpoint that it 
permits a blending of all of the physical sciences 
into a single world view, which grows in unity with 
the years, and with study. A glance at the table of 
contents of the present volume will reveal what may 
seem to the uninitiated reader a very heterogeneous 
assemblage of topics, but it is the hope of the writers 
that a perusal of the book its eh* will give a sense of the 
profound inner unity of all of these outwardly various 

It is the belief of the authors that a book of this nature, 
written in the light of the most recent discoveries, will 
find a welcome amongst the scientific laity, as well as 
with scientific or philosophic workers in general whose 
special fields are perhaps somewhat removed from that 
of theoretical physics. At the moment of writing there 
is no book available dealing with the whole modern 
theory of matter and energy in either an elementary or 
an advanced fashion, and treating it as a unit. Many 




admirable treatises on portions of the field are of course 
obtainable. For a considerably more advanced, yet 
not very difficult, discussion the reader is referred to 
three books which together cover the ground fairly 
thoroughly, o/z., The sixth edition of Nernst's "Theoret- 
ical Chemistry," the second of Campbell's "Modem 
Electrical Theory," and Rutherford's "Radio-active 
Substances and their Radiations." Specific references 
to other works are given at the end of each Section of 
Part n of the present book. 

Something must be said in explanation of the arrange- 
ment of the book. It consists of two parts, the first 
giving a rapid survey of the entire subject, outlining the 
fundamental conceptions and emphasizing then* most 
significant applications only, while the second retraces 
the same general field in a slower and less connected 
way, in order to consider details omitted hi the more 
cursory treatment. The second part is divided into fifty- 
six sections, each of which is numbered and referred to 
by its number in the appropriate connection in Part I. 

The book may be read in various ways according to 
the purposes or pleasure of the reader. If he is interested 
only to acquaint himself with the fundamentals of the 
modern theory through a quick, general sketch he may 
read Part I continuously and omit Part n altogether. 
If, on the other hand, he is already familiar with these 
essentials he may prefer to reverse the procedure and 
omit Part I. Part II although definitely divided by topics 
nevertheless forms a fairly continuous discussion. In 
general, however, the best method of using the book will 
probably be to read both of the sections in parallel, refer- 
ring to each Section in Part n as its number appears in 
the text of the first part. It is believed that this method 
of study will encourage the type of attitude which is re- 



quired to give the subject the greatest clearness in the 
reader's mind. It is obvious that the structure of Part 
n permits its ready use for purposes of special reference, 
such as may arise, for example, in connection with school 
courses in elementary physics and chemistry. 

The basis of Part I is to be found in a series of articles 
contributed in 1911 by D. F. Comstock to the " Science 
Conspectus," the journal of the Massachusetts Institute 
of Technology Society of Arts. Somewhat to his surprise, 
there was a wide demand from various sources for fur- 
ther copies of these articles, and hence it seemed worth 
while to publish them in book form, together with a more 
complete discussion of the same subject. The articles 
have been amplified and brought up to date by their 
original author. 

At Professor Comstock's suggestion, I undertook the 
writing of Part II, which provides the more elaborate 
treatment just mentioned. 


Boston, Mass. 









Their Size; Their Shape; The Different Kinds of 
Atoms; The Tendency Shown by Atoms to Form 
Groups; Elements and Compounds; Chemical Ac- 
tion; Permanence of the Atom; General Forces of 
Attraction Between Atoms and Between Groups of 


The Motion of the Molecules; Molecules are Per- 
fectly Elastic; Solid, Liquid and Gas, The Causes 
of Their Differences; The Brownian Movement and 
the Visibility of Heat Motion ; A Model of a Liquid ; 
A Model of a Solid; How Friction Causes Heat; 
Why " Evaporation Cools "; The " Absolute Zero "; 
The Heat Energy in Bodies. 


Its Size; Its Weight; Its Shape and Structure; The 
Two Electricities; Both Kinds of Electricity Abun- 
dant hi all Bodies; Electrons Negatively Charged; 
Atoms and Electricity; Negative Charge means " Too 



Many" Electrons, Positive Charge "Too Few"; 
The Electric Current; The Action of a Battery or 
Dynamo; "Free Electrons"; The "Evaporation" 
of Electrons. 

Electrons and Chemical Action; Electrons and Light; 
The Absorption of Electric Waves; The Reflection 
of Electric Waves; The Speed of Electric Waves in 
Different Bodies. 


The Connection of Electricity with Magnetism; The 
Deflection of Electrons Caused by Magnetism; The 
Action of a Dynamo; Permanent Magnetism; The 
Effect of Magnetism on Light. 

RADIO-ACTIVITY ................ 34 

The Three Rays; The Beta Rays; The Alpha Rays; 
The Gamma Rays; The Cause of Radio- Activity; 
Successive Disruptions of the Atoms of Radio-Active 
Substances; Radio- Activity not a Chemical Change; 
Intra- Atomic Energy; The Quantity of Intra- Atomic 
Energy; The Radio-Active Elements; Are all of the 
Elements Radio-Active? The Evolution of the Ele- 


General Principles; Evidence for Orderly Structure 
in the Atom ; Spectral Lines. 



Recent Advances Concerning the Atom; Atomic 
Numbers; The Quantum Theory; The Similarity 
of all Forms of Radiant Energy; X Rays. 

XL ATOMS AND LIFE ......... . ..... 60 









A brief statement of the history of the subject. 

The thickness of the thinnest known films of matter; 
Calculations based on the volume occupied by the 
atoms, on chemical deposition caused by the electric 
current, on the speed of ions, heat conduction, etc.; 
Agreement of the differently obtained results amongst 

Can atoms be seen? Nature of Colloids. 


Atoms probably spherical; Means of showing this; 
The " solar system " idea of the atom. 



Table of the elements, their symbols, atomic weights, 
general properties, and dates of discovery; Table of 
the radio-active atoms; Methods of ascertaining the 
relative weights of atoms, from chemical analysis, from 
the volumes occupied by gases ; How the volume of an 
atom is related to its weight, 



Systematic resemblances between different elements; 
The principle of the Periodic Table, "families" and 
"series" of elements; Our knowledge of elements as 
yet undiscovered; Defects in the Periodic System; 
The probable meaning of the system; Prout's Hypothe- 
sis: Helium and the Nucleus Theory; Isotopes; Meta- 
neon; The Table itself. 



The multitudinous compounds of carbon; " Isomers " 
and structural formulae ; Proof that our conceptions of 
molecular structure are correct in the case of " ben- 
zene," as an example; Molecules of single elements; 
How the shape of crystals depends on that of the mole- 
cules composing them. 



What determines these properties; Meaning of color; 
Individuality of molecules ; Allotropism ; Recent ideas 
concerning the basis of chemical individuality. 


The types of chemical change and the way in which the 
chemist represents them. 


Probable relation between gravitation and the attrac- 
tion between individual molecules and atoms; De- 
pendency of the forces of cohesion, etc., upon those of 
chemical affinity, and of the latter upon the forces within 
the atom itself. 


The nature of this theory and the ideas it is based on ; 
The idea of probability and the use of averages in mo- 
lecular physics ; Individuality in the molecular world. 




The temperature of a body is proportional to the " ki- 
netic energy " of its molecules ; Relative speeds of 
heavy and of light molecules at the same temperature ; 
The actual calculated speeds of certain molecules. 



Definition of "mean free path" in a gas; Properties 
of a gas affected by size of this path; Its length about 
one one-millionth of an inch under ordinary conditions. 


Its cause and mechanism. 

16. SOUND 101 

The structure of a sound-wave ; How it is set up and 
how it travels; Similarity between sound- and heat- 


Explanation of the fact that heat disappears when a 
body melts or vaporizes. Why solids soften when 
heated; Cause of the " surf ace tension" of liquids; 
The mechanism of evaporation. 



Definition of the "critical point" of a liquid; Change 
in latent heats and surface tension near critical point 
and reason therefor; What "boiling" means on the 
molecular theory. 


TIONS 106 

Why the pressure exerted by a gas increases with its 

degree of confinement, and with rise in temperature, the 

laws of Boyle and of Charles; Absolute Zero and the 



principle of Gay-Lussac; Explanation of the law of 
Avogadro ; Effect of volume of the molecules and their 
mutual attractions upon the laws of gases, the formula 
of Van Der Waals. 


Why dissolved substances obey the same general laws 
as gases. 


The cause of differences in the heat conductivity of 
solids, liquids and gases; The part played by "free 
electrons " in the conduction of heat. 


Method of studying the Brownian movement; Specific 
results verifying the kinetic molecular theory. 


The difference between crystalline and "amorphous" 
bodies; The crystal as the unit of structure of matter 
just above the molecule ; Crystal structure as studied 
by X rays; Liquid crystals. 


Although for a given temperature all of the molecules 
do not move at the same speed, most of them tend to 
have at least approximately the average speed for all; 
How this fact explains the manner in which the rapidity 
of evaporation of liquid increases with temperature; 
Similarly with respect to the pressure exerted by the 
resulting vapor ; Why a liquid and its vapor maintain the 
same temperature in spite of the "cooling effect of 



Definition of the "total heat energy" of a body, and of 
"specific heat"; Du Long and Petit's law of "atomic 
heats " and its explanation; Explanation of the constant 
relation between the atomic heats of solids and of gases; 
Deviations from these rules and their probable signifi- 



J. J. Thomson's work on the "cathode rays"; How 
Thomson determined the mass and charge of the elec- 
tron; Counting electrons by the use of a fog; How the 
size of the electron can be calculated; Its substance 
and its structure. 



All physical events probably determined by such forces 
in the last analysis. 


ATOMS 125 

Definition of an "ion," and how ions are produced; 
Energy required to drag an electron from an atom; 
How electrons and ions of different kinds act on one 
another; Rules for such action. 



The "Hall Effect," why magnetism deflects an electric 
current; Nature of electrical "resistance"; Signifi- 
cance of "amperage " ; Why the best electrical conduc- 
tors are also the best heat conductors, and why metals 
are in general superior to other substances hi these 
respects ; The motion of electrons in a wire is opposite 
in direction to the "current." 



Ions carry electricity in these substances; Nature of 
electro-chemical action, or " electrolysis." 



Mechanism of this transmission. 


The various "affinities" of different substances for 
electrons; the operation of a "thermopile" explained 
on the electron theory; The elements arranged in order 
of their affinities for electrons. 


Electro-negative and electro-positive elements; Ions 
and electrons in chemical action; Electro-negativity or 
positivity only a relative conception; How atoms of the 
same species can be attracted electrically; Nature of 
chemically "inert" elements. 


How water can "ionize" substances which dissolve in 
it; Definition of "electrolytic dissociation"; Motion of 
the ions in a solution under the influence of electrical 
force; How it is proven that water dissociates dis- 
solved substances, effect on boiling and freezing 


Definition and cause of valency. 


The complexity of the changes involved in chemical 
action; Chemical change depends on the chance col- 
lision of molecules; Explanation of the fundamental 
"law of chemical mass action" on this basis; Rever- 
sible and irreversible chemical processes; Chemical 
equilibrium and its kinetic nature. 



Heat and chemical change; How electric current and 
light can be generated by chemical action; Nature of 
"chemical energy." 



FORCE 146 

Present status of the "aether" theory; Definition and 
nature of a line of electrical force; Formation of 
"kinks" in such lines; Light not a continuous wave- 


General nature of the theory of the effect, and results 
of its application to the phenomena; The Stark Effect. 



Temperature radiation; Why the light from a glowing 
body is whiter the hotter the body; The law connect- 
ing wave-length and energy of emitted light with tem- 
perature, and its general explanation in terms of the 
electron theory ; The emission of light by gases ; loni- 
zation and the production of "line spectra"; Spectral 


The complete spectrum, including all electrical waves; 
Position of visible light, "ultra-violet," "infra-red," 
heat, "Hertz waves," X rays, etc., in this spectrum; 
Velocity of light; Actual lengths and frequencies of 
light and other electrical waves. 



How color is produced by absorption; Explanation of 
the "selective absorption" of light; Basis of the sen- 
sations of color; Production of color by reflection. 


How a column of light is bent in passing from air into 
glass; Definition of "dispersion" and statement of 
the law governing it; Relation between the index of 
refraction of a substance and its "dielectric capacity." 




How it was shown that the motion of an electrical charge 
causes magnetism. 



How the experiment is performed. 


The two kinds of magnetism; How permanent magnet- 
ism is possible. 


The Work of Becquerel and the Curies; The "Radium 
series"; The law of decay of radio-active substances; 
Their position in the Periodic Table. 


The differential effect of magnetism on the rays ; Pene- 
trating power of the beta rays. 



Description of the experiment. 


Relation of the gamma rays to the beta rays and the 
disruption of the radio-active atom ; Secondary gamma 


The great stability of the atom; Relation of intra- to 
inter-atomic forces and energies. 


The work of Campbell. 




The variability of the line spectra of the elements; The 
spectra shown by the hottest stars are the most imper- 
fect; Lockyer has shown that the very hottest stars con- 
tain only the simplest elements; Meaning of these 


Thomson's theory and its partial explanation of the 
mystery of the Periodic Table; The modern "Nucleus 
Theory " ; The empirical basis of this latter theory ; The 
number of electrons in the atom; Isotropism; Atomic 
numbers ; The hydrogen atom, its constitution and the 
basis of its line spectrum as deduced from the " Quan- 
tum Theory " of light. 


The nature of the theory; The photo-electric effect; 
The relation between the "frequency" and energy of 
light quanta; The conditions of the absorption and emis- 
sion of quanta; The explanation of the low values of 
specific heats near absolute zero temperature ; Planck's 
original reason for propounding the theory; The broad 
significance of the theory; The doctrine of entropy and 
its basis in the theory of probabilities; Entropy and 


The origin and nature of X rays; Characteristic X rays; 
Why Xrays penetrate "opaque" bodies; The cor- 
puscular properties of X rays; The reflection and 
"diffraction" of X rays by crystals; New light on 
crystal structure. 


Vital phenomena are consistent with an explanation in 
terms of atoms, molecules and electrons. 

INDEX 196 




Sir Joseph J. Thomson Frontispiece 

Fig. 1. The Relative Sizes of Atoms and Molecules ... 3 

Fig. 2. Relative size of Molecules and Visible Particles . 4 

Fig. 3. Water Molecules Facing page 4 

Fig. 4. Molecules of Steam 6 

Fig. 5. Atoms of a Liquid 6 

Fig. 6a. Formulae of some Common Organic Compounds . . 8 

Fig. 6b. Formula of an Azo-dye 9 

Fig. 7. Atoms of a Solid Facing page 10 

Fig. 8. Gas Molecules 12 

Fig. 9. Vapor Molecules at the Surface of a Liquid .... 15 

Fig. 10. The Constitution of a Simple Molecule 28 

Figs, lla, b. The Radio-Active Elements and their Relation- 
ships and Rays 36, 37 

Fig. 12. Five Isomeric Hydrocarbons having the Constitution 

C 6 H 14 79 

Figs. 13a, b. Benzene and its Chlorine Derivatives .... 81, 82 

Fig. 14. Models of Tartaric Acid Molecules 84 

Fig. 15. Crystals of "Right" and "Left" Tartaric Acids . . 86 

Fig. 16. Diffusion Paths 100 

Fig. 17. "Distribution Curve" for Molecular Speeds ... 116 
Fig. 18. Vacuum Tube to show the Action of the Cathode 

Rays 121 

Fig. 19. How the Cathode Rays May be Bent by a Magnet . 122 

Fig. 20. The Forces Acting Between Ions, Atoms and Electrons 127 

Fig. 21. A Thermo-Electric Circuit 134 

Fig. 22. Showing the Manner in which Two Neutral Aggre- 
gates of Electrical Particles may attract eath other. 138 
Fig. 23. To Show how Radiation is Produced by Stopping the 

Motion of an Electrical Particle 148 


Fig. 24. The Zeeman Effect 160 

Fig. 26. Curve Showing the Relative Intensities of Radiation 
of Different Wave Lengths Emitted by Solid Bodies 
at Various Temperatures 153 

Fig. 26. The Direction of the Magnetic Forces about a Moving 

Electrical Charge 162 

Fig. 27. Structural Plan of a Simple Crystal 192 


I. Line Spectrum of Iron Facing page 42 

n. Cavendish Gravitation Apparatus .... " " 92 

in. Thomson Cathode Ray Tube " " 122 

IV. The Deflection of Cathode Rays by a Magnet. " " 162 

V. Coolidge X Ray Tube " " 190 





During the last two decades there has been a very 
great advance in our knowledge of the ultimate constitu- 
tion of matter. (1) The older ideas which prompted the 
contemptuous phrase " gross matter" are inadequate 
to represent the extraordinary complexity and delicacy 
of structure which have since been revealed. The end 
is of course not yet, but throughout all this advance 
there has been singularly little in former ideas which 
had to be considered totally wrong. They were right as 
far as they went, although insufficient, and so it proba- 
bly is with our present ideas respecting the structure of 
things; they will doubtless appear crude in the light of 
future knowledge but in a general way they are proba- 
bly right as far as they go, and hence are worthy of our 



According to the modern theory of matter all bodies 
are complex structures composed of small particles called 

NOTE: The full-face numbers inserted in the text at various 
points refer to the Sections of Part II in which related subjects are 
discussed (see Preface), or in which further details are given on 
the same subject. 



atoms, together with still smaller particles known as 
electrons. If, therefore, we were familiar with the laws 
of action of atoms and electrons we should understand 
completely all the physical phenomena in nature. The 
atom, as we shall see later, is a much more complex 
structure than the electron, so that atoms and elec- 
trons are not quite on a par as regards classification, 
except from the introductory point of view, from which 
we begin discussion. 

As a third fundamental entity, there should be men- 
tioned the energy associated with atoms and electrons, 
but for the present this will be left out of consideration. 



Size. Atoms are minute particles each about one 
three-hundred-millionth of an inch in diameter (2). If 
the earth were made up of base-balls it would be a fair 
model of a drop of water made up of atoms. The most 
powerful microscope known, used under the best condi- 
tions, would enable us to see an object approximately 
two hundred atoms in width. Single atoms are, there- 
fore, totally invisible, and their properties cannot be 
found out by direct inspection (3). 

Shape. Not much is known as regards the shape 
of the atoms, but in general they behave as if they were 
not very far from spherical (4). 

Different Kinds. We are now acquainted with about 
one hundred different kinds of atoms, that is, different 
species. The individual atoms in each species are, how- 
ever, exactly alike, or have so nearly the same properties 
that under most conditions there is no difference in the 



action of the individuals. Atoms of different kinds differ 
in size and still more in weight. 

At present there is no agreement as to the difference 
in size of the various kinds of atoms. On the basis of 
certain calculations from coefficients of expansion, some 

Fig. 1 

This diagram is intended to give an idea of the relative magnitudes 
of atoms and molecules. However, the drawings are only symbolic, as 
the dimensions have been calculated on the assumption that the mole- 
cules are spherical, which cannot be strictly true. It will be noticed that 
the smallest atom (that of hydrogen) differs only slightly in size from 
the largest atom (that of uranium). The starch molecule is probably 
one of the largest which exists and it will be seen that, according to the 
diagram, it is very much larger than the largest atom or than the mole- 
cule of sugar. The relative weights of the particles represented are as fol- 
lows: Hydrogen, 1; Uranium, 239; Sugar, 366; and Starch, not accu- 
rately known but probably about 25,000. A molecule of ordinary alcohol 
weighing 46, would be slightly larger than the uranium atom. 

investigators believe that the sizes of the different kinds 
of atoms are in the same order as their weights. Ac- 
cording to this view, therefore, the lightest atom, hydro- 
gen, is also the smallest; and the heaviest atom, uranium, 
is also the largest. The atom of uranium is about 240 
times as heavy as the atom of hydrogen, whereas it has 



only about two and one-half times as great a diameter. 
If this view is correct, we might represent an atom of 
hydrogen by a wooden ball the size of a pea, and an 
atom of uranium by a lead ball the size of a cherry. 
Representatives of all of the other atomic species would 
then be arranged in a complete series from the small 
wooden ball to the large lead one. 

Fig. 2 

The molecule represented in this diagram is the starch molecule of 
Figure 1, very much reduced in scale. It is not certain that the starch 
molecule is the largest which exists, but it is very far from being the 
smallest. The microscopic particle which is represented is the small- 
est which can actually be seen under the most powerful microscope. 
Particles nearly as small as the starch molecule can be seen indirectly 
by means of the ultra-microscope. (See Section 3.) 

Although the individual atoms of one kind are, with 
certain modern reservations, all alike, those of different 
species have decidedly different properties, and this 
difference hi property is what gives variety to the phys- 
ical world as we see it. The atoms of one species are 



so definite, unique, and characteristic in their actions 
and properties that they give one the impression of a 
delicacy and complexity of structure suggestive, almost, 
of the complexity of personality. There are subtle resem- 
blances between one species and another with regard to 
one -property, and marked differences with regard to 
another (6). Hence one should be very careful to realize 
that when, for reasons of analogy, we represent an atom 
as a ball of wood or a ball of lead we are representing it 
only in the vaguest general way, and are totally ignoring 
its complexity and individuality. 

Tendency to Form Groups (Molecules). Atoms tend to 
form groups known as molecules. The atoms in a mole- 
cule adhere with considerable force and some molecules 
can be broken up only with the greatest difficulty. These 
groups have a definite individuality and unless acted on 
from the outside they are apparently permanent. The 
same atoms may be grouped into quite different molecules 
just as the same bricks may be used to build a church 
or a jail, or the same letters used to form altogether 
different words. The individuality of a molecule is per- 
haps best appreciated by thinking of the individuality 
of a word. A word, though consisting solely of letters, 
has a definite unity of its own. A molecule made up of 
atoms is just as definite an aggregate (7). 

As a help toward a concrete conception two drawings 
are given in Figures 3 and 4. The first represents sym- 
bolically water molecules, each consisting of two hydro- 
gen atoms and one oxygen atom, in the closely crowded 
state known as liquid, and the second the same mole- 
cules in the more dispersed state known as vapor 

Elements and Compounds. When the atoms making 
up the molecules of a substance are all alike, that is when 



they belong to the same species, the substance is called 
an "element." An element, therefore, is composed of 
only one kind of atom. A compound is a substance the 
molecules of which are made up of more than one kind 
of atom. 

Figure 5 represents symbolically a liquid element, 
Figure 3 a liquid compound. "Oxygen," "hydrogen," 
"carbon," "lead," "copper," are names of some of the 
elements, and they are, therefore, names of atomic species. 
"Water," "salt," "sugar," and "carbon-dioxide" are 
names of compounds, and hence are the names of 
molecular species. 

The water molecules in Figure 3 are seen to consist of 
one large atom and two smaller ones in a group. The 
large atom is an oxygen atom. The two smaller ones are 
hydrogen atoms and the group as a whole is a water 
molecule. It is therefore true to say that a water mole- 
cule is the smallest particle of water that it is possible to 
have, for if it is further broken up it is no longer 
water. These drawings are in no sense other than 

As a moment's thought will show, thousands upon 
thousands of different kinds of molecules are known. 
Some, such as the water molecule, are relatively simple 
and composed of a few atoms, and some, such as the sugar 
molecule, are very complex. (See Figure 6.) In the par- 
ticular case of the sugar molecule the number of atoms 
is forty-five. 

A small crystal of "granulated" sugar is, therefore, a 
solid mass consisting of hundreds of millions of sugar 
molecules, that is hundreds of millions of definite, co- 
herent groups of atoms, each group containing twelve 
atoms of carbon, twenty-two of hydrogen, and eleven of 



The definiteness of molecular structure must not be 
forgotten. One of these sugar molecules might be com- 
pared to a word of forty-five letters, for if a single atom 
were removed from the group, or if a single atom had its 
position markedly changed, the group might still be a 
molecule, but it would not be a molecule of sugar, and a 
vast mass of such modified molecules would not make 
up a crystal having the same properties as the one with 
which we started (8). 

Chemical Action. Chemical action is the name given 
to the process in which the groups known as molecules 
are either formed or destroyed. When a substance is 
burned or when an acid "eats" a metal the action in- 
volves the formation of new molecules, because of the re- 
arrangement of the atoms, and therefore the production 
of new substances. By an inspection of Figure 3 it will 
be clear that if two of the "large " oxygen atoms could be 
separated from their respective water molecules, to re- 
combine as represented in Figure 8, there would be left 
behind, four of the "small" hydrogen atoms of Figure 3 
(two from each decomposed molecule), and these would 
adhere in twos and would form two hydrogen molecules. 
When thousands of molecules were thus broken up the 
complete process would be called the decomposition of 
water into oxygen and hydrogen. It is easily accom- 
plished in the laboratory. 

It is clear that when complex molecules are present it 
is possible for the rearrangement which takes place to be 
very complicated indeed. There are often, also, several 
possibilities of rearrangement, each resulting in a dif- 
ferent set of substances as a final outcome. 

Outside conditions such as temperature, pressure, etc., 
have marked effects on the results of chemical reactions. 

Chemical action, therefore, always implies the break- 



H H 
Ethyl Alcohol H C C O H 

Acetic Acid H C C= O 



H O C H 

Grape Sugar H O C H 
(dextrose) H C O H 

H O C-H 
0=C H 
Fig. 6a 


NOTE. The formula of ordinary cane sugar (or saccharose) is not definitely 
established, but probably consists of two groups of atoms similar to that for 
dextrose, combined with a molecule of water. 



H H H H H H 

c c U U 

/. \ / .\ S \ S \ 

H-C C C-N=N-C C-C C-N= 

c c x c=/ x c=c x 

Y A A A A 

H O=S=O 




H C H 

H H H 

U A A 

=K-/ Vp-f-/ V % C-H 
C=C X H-C C C-H 



o c-c I 

Na O S C C O H 

\ / Na 

O C= C 

H C C H 



Fig. 6b 


ing up or forming of molecules, and in general it means 
both (9). 

Permanence of the Atom. Although every one of the 
thousands of "chemical reactions" which are daily going 
on hi the world involves the formation or decomposition 
of molecules, no way has yet been found to change an atom 
of one species into one of another species. To find such 
a way was the hope of the alchemists but it was never 
realized. We shall see later that the atoms are probably 
not absolutely permanent but that the mysterious forces 
which preserve their integrity are so much greater than 
the forces which hold them together in molecules, that 
as yet it has not been found possible to shatter them 

The atoms have great family attraction and it is the 
business of the chemist to make use of this attraction in 
the service of man, but the instinct, we might say, of self- 
preservation is so vastly greater hi the atom than its 
group-forming tendency, that although it submits to the 
breaking of family ties it will not allow its own individual- 
ity to be tampered with. 

To state that it will never be possible to break up atoms 
artificially would of course be folly, but we can say at 
present with considerable certainty that the disruption 
of atoms must involve forces of an entirely different 
magnitude from those called " chemical," which are 
associated with the breaking up of molecules into 

We shall see later that there appears to be going on in 
nature a spontaneous decomposition of the atoms of 
radium and certain other substances, but thus far it has 
been found quite impossible artificially to influence this 
spontaneous disruption to the slightest degree, and there- 
fore although we might call the process " natural al- 



-i^. ^aafc lip -, III 




chemy" it cannot be called alchemy in the ordinary 
sense of the word. 

General Forces of Attraction. Just as atoms have 
forces of attraction which hold them together in mole- 
cules, so molecules attract each other and tend to form 
the large aggregates which we call " objects." These 
forces are in general weaker than the forces between the 
atoms (10). They are great enough, however, to account 
for the relatively strong cohesion of solid bodies and the 
weaker cohesion of liquids. 



The Motion of the Molecules. It is now believed, and 
the odds amount almost to certainty (11), that all atoms 
of all substances are in ceaseless motion to and fro. 
This motion is what we call the heat of a body. 1 The 
more violent the motion the hotter the body (12). It is 
perhaps a pity that the motion picture art is not de- 
veloped to a point which would enable us to embody 
this violent vibration in the accompanying figures, so 
we must request imagination to aid incompetent art 
and to endow every atom or molecule of Figures 3 to 8 
with a rapid motion; a motion which, like the modern 
idea of freedom, is limited only by the equal rights of 
all the other atoms. 

Thus in the liquid and solid states (Figures 3, 5 and 7) 
the crowding is so close that "the rights of others" 

1 Strictly speaking, it is the energy of the atomic or molecular 
motion which constitutes the heat of a body. The distinction 
between atoms, molecules and electrons is not important when 
heat phenomena are being described in a general way since it is 
probable that all of the particles share in the motion. 



allow only vibration through a very limited distance, 
while in the gaseous state (Figures 4 and 8), the motion 
consists of a straight line flight until by chance there is 
an encounter with another molecule. When this occurs 
there is a rebound and then another flight. The distance 
between the molecules is so small and their speed is so 
large that literally billions of these impacts are occurring 
every second in even a cubic inch of gas (13). 

Molecules Have No Friction. It is necessary in order 
that this motion should continue indefinitely that the 
atoms or molecules l be considered frictionless. At first 
sight this seems an improbable hypothesis, but when the 
nature of friction is understood from the present point of 
view the assumption is seen to be justified. A number of 
billiard balls put on a table and set going would bound to 
and fro for a short time and gradually come to a stop, but 
this is because at every impact part of the energy of the 
balls' motion is wasted in the form of heat, that is, the 
balls are actually a trifle warmer after striking each other 
than before. Now from the present point of view, as we 
have just seen, "warmer" means more rapid molecular 
motion, so that the billiard balls gradually slow down 
and stop because their motion is gradually transformed into 
the invisible motion of the molecules which compose them 
and surrounding objects. Thus friction exists between 
visible objects solely because of the existence of the 
molecules which go to make them up, these molecules 
absorbing the motion. 

From this it is clear that for two molecules to waste 
energy in impact after the manner of two billiard balls, 

1 The argument here presented applies strictly only to the 
" ultimate particles " of matter, whatever these may be. How- 
ever, no very important inaccuracy can result from identifying 
these ultimate particles with atoms, molecules and electrons. 



I f 


it would be necessary for them to be composed of still 
smaller "molecules." Hence, the ultimate particles 
themselves cannot possess friction in the ordinary sense. 

As a matter of fact we shall see later that molecules 
do lose energy, not in a way similar to the friction of large 
objects, but by radiating it in the form of heat waves. An 
analogy for this radiation of energy may still be found 
among billiard balls, for if they, the table, the cushions 
against which they strike, and the air around, were all 
perfectly elastic and frictionless, the balls when set going 
would keep then* motion for a far greater time than they 
actually do, but they would not continue to move indefi- 
nitely. This is because at each impact between two balls 
there would still be a sound, the "crack" with which 
we are all familiar, and this would carry energy away. 
This loss of energy by sound is vaguely similar to the 
radiation of heat energy by the molecules of a substance 

Solid, Liquid and Gas. All substances which do not 
decompose (that is, all substances the molecules of which 
do not break up) on heating, are capable of existing in 
three states, the solid, the liquid and the gaseous. A 
solid when heated above its melting point becomes a 
liquid and a liquid through the process of evaporation or 
boiling changes into a gas. It is also possible, at cer- 
tain temperatures and pressures, for a solid to pass 
directly into the gaseous state and vice versa. 

From the present point of view the cause of these 
changes is readily seen. We have said that heating a 
body means increasing the violence of its internal vibra- 
tion. Now it is easy to imagine that when in a solid sub- 
stance this vibration comes to exceed a certain amount 
the atoms or molecules will no longer be able to adhere 
in orderly arrangement but will be forced farther apart 



by the motion and, although not completely out of the 
influence of each other's attraction so that they become 
totally dispersed, still are so far apart that they wander 
about at random, like the frantic members of a mob. 
Under these circumstances rigidity no longer exists and 
the substance is liquid (16). 

When the vibration gets still more violent, i.e., when the 
liquid is further heated, the number of molecules at the 
surface of the liquid which escape into the surrounding 
region becomes very large. The molecules which escape 
do so because amid the random vibration they happen to 
have a speed sufficient to carry them up beyond the at- 
traction of the other molecules of the liquid. These 
molecules in the space above the liquid constitute a 
gas (17). 

In the case of a solid or liquid the heat motion takes 
place through a very short distance and is then reversed, 
and therefore we may speak with propriety of the motion 
as a "vibration" of the molecules or atoms. In the case 
of a gas, however, the motion is different, each molecule 
travelling practically in a straight line until by chance it 
encounters another flying molecule. In such a gas as 
air (which is mainly a mixture of oxygen gas and nitrogen 
gas) a molecule travels on the average through a distance 
several hundred times its own diameter before it strikes 
another (14). 

The existence of this atomic and molecular motion is 
at the very heart of the modern conception of matter. A 
somewhat extended analogy will, therefore, not be out 
of place. 

Suppose that into a room are thrown at high speed, 
through an open door, ten thousand tennis balls, and that 
the door is then closed. It is clear that the balls will for 
a time bound back and forth among themselves, striking 



the walls and each other. If, now, we make the ideal 
assumption that the walls of the room and the balls are 
perfectly elastic, that is, that there is no energy lost at 
any of the impacts, it is clear that the bouncing of the 


A A- -& - 

/ \ 

-x ' \ A 

. >' \ l'\ 


! A * 

Fig. 9 


As explained in the text, the vapor which rises from the surface of any 
liquid consists in molecules which are shot through the film of surface 
attraction. Slow-moving molecules may penetrate the liquid surface but 
be returned to it once more by the forces of attraction. Fast-moving 
molecules, however, may escape permanently. The paths described by 
molecules of both sorts are illustrated above. A is the limit at which 
the attraction ceases to be effective for a molecule moving sufficiently 
fast to reach this line. B is the liquid surface. 

balls will go on indefinitely. Such a room with the balls 
will represent in a general way a small vessel filled with 
a gas. 1 

Some of the well-known properties of gases follow in 
the simplest way from a consideration of the above model 
of a gas. It is obvious that the walls of the room will be 

1 The analogy here given may appear crude, but an actual model 
on the general principle outlined, using small steel balls, has been 
constructed by Professor Northrup of Princeton. When in 
action, this model exhibits all of the fundamental properties and 
laws of gases. 



bombarded by the flying tennis balls. The walls will, 
therefore, feel a thrust, that is, they will have an outward 
pressure acting upon them. This corresponds to the well- 
known pressure of any gas confined in a closed vessel. 
Moreover, this pressure due to bombardment will be 
greater if the speed of the flying balls is greater, and 
hence in the case of a gas we should expect the pressure 
to increase with the temperature, that is, with the aver- 
age energy of motion of a molecule. It is a well-known 
fact that the pressure of a gas does increase in this way 
(18), (19). 

Again, returning to our model, it seems fairly reason- 
able to suppose (and can, in fact, be proved mathemati- 
cally) that if the balls hi one hah* of the room have on 
the average a higher speed than the balls in the other 
half, there will be a gradual slowing down of the one and 
a speeding up of the other until all of the balls have the 
same average speed. This corresponds to the gradual 
equalization of temperature which goes on in any vessel 
containing a gas, when at the start one part of the gas is 
at a higher temperature than another. This transmission 
of energy in a gas is what is called heat conduction (20). 

The Brotonian Movement and the Visibility of Heat 
Motion. If one of the tennis balls above mentioned were 
much larger and heavier than the rest it would be found 
to move on the average much more slowly. In the case 
of a gas, therefore, heavy molecules move on the average 
more slowly than light ones. As we pass to heavier and 
heavier molecules or to larger and larger particles of 
some foreign substance immersed in the gas, the average 
motion of the particles considered becomes less and less, 
until it disappears into the imperceptible. Now the ques- 
tion arises whether it might not be possible to detect the 
motion of particles so large that they could be seen with 



a microscope. If this were possible we could obtain a 
direct view of the heat motion of the gas, for although we 
should not be able to see the motion of single molecules 
we could see the closely related motion of a very much 
larger particle. 

Now, as a matter of fact, this continuous random 
motion of all very small solid particles floating hi a gas 
or a liquid is a well-known phenomenon and is called the 
Brownian Movement. It is so common that biologists 
have to learn to distinguish between the life-motions of 
bacteria which they are examining under the micro- 
scope and the "Broionian movements" which the bacteria 
have in common with all other small particles. 

This Brownian movement is an extraordinary veri- 
fication of modern ideas of heat for the verification 
goes farther than was stated above. It has recently 
been shown that the motion observed under the micro- 
scope is not only of the same *W but also of the same 
magnitude as that which is predicted mathematically 
from molecular considerations (21). 

A Model of a Liquid. If we imagine the above men- 
tioned tennis balls to have then- average motion slowly 
diminished, and if we remember that in order to repre- 
sent molecules the balls must have a slight attraction 
for each other, it will be clear that finally the balls will 
no longer fill the room as they did before but will divide 
themselves into two groups. There will be a layer of balls 
on the floor adhering more or less closely together, al- 
though still vibrating among themselves, and above this 
layer there will be flying at random the balls which we 
have already taken to represent a gas. The layer on the 
floor might be a foot or two thick, depending on the num- 
ber of balls present, and they would give us the impres- 
sion of a ceaselessly squirming mass. 



This dense layer of agitated balls on the floor repre- 
sents the surface of the liquid and the flying balls in 
the remainder of the room represent the gas or vapor 
which always exists above any liquid confined in a closed 

It will be clear hi a general way from this model why 
"heat expands" in the case of a liquid, for as the violence 
of vibration increases (and this corresponds to a rise in 
temperature) the closely adhering balls representing the 
liquid will, on the average, be forced farther apart and 
the total volume of balls will appear to fill more space. 

In a few rare cases a liquid expands on being cooled. 
In these cases we must imagine that the molecules 
are not simple spheres but have more complicated 
shapes. A change in temperature may therefore result 
in a different fitting together and unexpected volume 

A Model of a Solid. The model above discussed 
probably represents the truth in a general way as re- 
gards a liquid, a gas, and the relation between the two, 
but we cannot be so sure in the case of a solid. 

The tennis balls, if they are to represent molecules, 
must not be perfectly spherical. Let us suppose them 
to be egg-shaped. Let us suppose too that the internal 
motion of the balls on the floor be diminished more and 
more and that after a while there is a tendency to form 
orderly arrangements which persist. They will still be 
vibrating somewhat, but if one of the balls has its sharper 
end turned in one direction at present, it will be turned hi 
the same direction at a later period. It is as if a net-work 
of elastic threads fastened the balls together. They are 
still capable of vibrating to and fro, but any individual 
maintains permanently a certain position in the total 



Such a model probably corresponds to a substance in 
the solid state, although, as has been said, this is not 
certain at present. There may be some other subtle 
difference between a liquid and a solid, but this is per- 
haps the principal one. One reason why we are not 
very sure that permanent orderly arrangement is the 
only difference between a solid and a liquid is because 
of the existence of orderly arrangement within liquids. 
Liquid crystals, as they are called, are known to exist and 
certainly are of extreme interest and importance (22). 

How Friction Causes Heat. It will be clear from the 
above considerations why the rubbing of one surface on 
another invariably causes heat. The molecules of both 
bodies are "stirred up" as it were, so that the violence 
of vibration is increased and this corresponds to a rise in 
temperature. The energy required to increase the vi- 
bration of the molecules comes from the work done in 
the rubbing. 

Why " Evaporation Cools." - That the evaporation of 
a liquid has a cooling effect is well known to everyone. 
Boys detect the direction of the wind by noticing the 
coolness of one side of a wet finger. From the above 
consideration the reason for this cooling effect is not 
difficult to imagine. From the surface of a liquid, mole- 
cules are constantly passing away to become part of the 
surrounding vapor. In the course of the random motion 
which a molecule at the surface of a liquid undergoes it 
may at certain times, as has been said, attain sufficient 
speed to enable it to break away from the attraction of 
its neighbors. (See Figure 9.) Since the deserters will 
always be molecules which at the moment possess greater 
speeds than the average, the liquid by evaporation is 
constantly losing some of its fastest moving particles. 
Such selective action will result in a gradual decrease in 



the average speed of those that remain. This corre- 
sponds to a cooling of the liquid (23). 

The "Absolute Zero." It has been said that from the 
modern point of view the violence of molecular or atomic 
vibration corresponds to the temperature of a body. 
Now if we imagine a body to be cooled indefinitely it is 
clear that sooner or later we should reach a point at 
which all vibration would have ceased, that is, when the 
molecules and atoms of a body would simply be packed 
together in an absolutely inert mass. Such a point would 
correspond to a temperature below which it would be 
impossible to go, because it is obviously impossible to 
have less vibration than no vibration. 

There are ways (see Section 18) of calculating in terms 
of degrees Fahrenheit this lowest conceivable tempera- 
ture although it cannot be completely attained in practice. 
It is approximately 459 below zero. This temperature is 
called "the absolute zero" and corresponds to the ab- 
sence of all heat. 

"Heat" and "cold" are consequently not symmetrical 
terms. "Heat" is molecular motion. "Cold" is the 
absence of molecular motion, that is, the absence of heat. 
It is therefore wrong to speak of "adding cold" to a 
body. We should say, "taking heat away." The value 
of ice in a refrigerator consists in the fact that it absorbs 
large quantities of heat from the objects put near it and 
not that it "gives out cold." 

The Heat Energy in Bodies. From the above con- 
siderations it follows that all bodies at room-temperature 
possess enormous quantities of heat and what we call a 
"hot body" is distinguished from a "cold body" by the 
fact that the first has a higher temperature than the hu- 
man body, and that the second has a lower temperature 



It may be worth stating that the amount of heat in a 
glass of water at ordinary temperatures corresponds to an 
amount of energy which, if utilized mechanically, would 
be sufficient to raise this water to a distance of thirty 
miles or more above the ground. Practically the same 
would be true of a piece of ice, since its total heat is only 
a little less than that of the water. 

It may also be worth mentioning that in the last few 
years temperatures have been reached in the laboratory 
which are only two or three degrees above the absolute 
zero. At such low temperatures some of the properties 
of matter, as we shall see later (Part II, Section 54), 
undergo remarkable modifications. 



We are now ready to consider the second fundamental 
entity, namely the electron. To quote from the first 
chapter, " according to the modern theory, all bodies are 
complex structures composed of small particles called 
atoms and still smaller particles known as electrons." 

So far as we know, all electrons are exactly alike. In 
this respect, therefore, they differ greatly from atoms, 
which, it will be remembered, exist in about a hundred 
different varieties. 

Size. In size the electron is very much smaller than 
the atom. The exact size is not known but it has a di- 
ameter of about one one-hundred-thousandth that of an 
atom. This means that if the average atom be repre- 
sented by a sphere one hundred yards in diameter, the 
electron, on the same scale, would be about the size of a 
pin-head. In other words, a large office building is not 



large enough to represent an atom if a pin-head is to 
represent an electron. 

Weight. The electron is much lighter than any known 
atom although hi proportion to its size it is much 
heavier. Although the atom is enormously larger than 
the electron, the lightest atom, namely that of hydro- 
gen, is only about two thousand times as heavy as an 
electron. 1 A short calculation shows, therefore, that 
the " density" of the electron is a million million times 
that of the atom. 

The minuteness of the electron may seem almost in- 
credible, but careful research leads almost inevitably to 
the conclusions stated, and the scientist must report 
what he finds. 

Shape and Structure. Practically nothing is known 
as to the shape or structure of the electron. There are 
indications, however, that it is spherical in shape and 
symmetrical hi every way (25). 

The Two Electricities. It will be remembered by those 
whose physics is not too distantly lost in the past that 
there are two "kinds" of electricity, " positive" and 
"negative." If a body is charged with electricity of 
one kind it repels all bodies having a similar charge 
and attracts all those having an opposite charge. In 
the familiar terms: "like charges repel each other; 
unlike charges attract each other." The attraction 
of unlike charges is the common phenomenon noticed 
when in cold weather a recently used comb is held 
near bits of paper. At present it seems not improbable 
that most of the phenomena in nature are due, in the last 
analysis, to electric attractions and repulsions (26). 

1 Strictly speaking it is the " mass" and not the weight that we 
refer to, but the term " weight" is in common use and of proper 



Both Kinds of Electricity Abundant in all Bodies. All 
bodies seem to possess enormous quantities of both 
positive and negative electricity, but usually it is in exactly 
equal amounts, so that one kind neutralizes completely the 
effect of the other and no electricity seems to be present. 
Charging a body with electricity is then to be considered 
as merely disturbing this balance by taking away or add- 
ing to the body a small amount of one kind of electricity. 
We shall see later that the electricity added or taken away 
appears hi the light of modern theory always to be the 

Electrons Negatively Charged. Each electron has a 
negative charge of electricity and this charge, consider- 
ing the size of the particle, is very great. Electrons are 
therefore, attracted towards all positive charges of elec- 
tricity and at the same time repel each other strongly. 
Modern research has made it probable that not only do 
electrons always possess a negative charge but negative 
electricity exists only in the form of electrons. That is, 
negative electricity and electrons are inseparable and the 
only way to give a body a negative charge is to put 
electrons on it or hi it. 

Atoms and Electricity. Since all bodies are made up 
of atoms, charging a body with electricity is the same as 
charging some of its atoms with electricity. Speaking 
now of " atoms" instead of " bodies," it follows from 
the above that no atom can be charged with negative 
electricity without putting one or more electrons on it. 
Each ordinary atom contains a number of electrons and 
enough positive electricity to exactly balance the negative 
electricity of the electrons. At present it appears that 
the positive electricity never leaves the atom, whereas elec- 
trons allow themselves to be taken away from or added 
to the atom with relative ease. 



Negative Charge "Too Many" Electrons; Positive Charge 
" Too Few." - Since it is probable that only negative 
electricity in the form of electrons is movable, an atom 
can be charged positively only by taking away some of 
the electrons which it normally possesses. This allows 
the positive charge of the atom (which it has perpetually) 
to predominate and produces the same effect as if posi- 
tive electricity had been added to it. Thus briefly, an 
atom contains normally a certain number of electrons 
and also positive electricity enough to neutralize exactly 
their negative charges. The atom is then "uncharged." 
If an electron is added to the atom from the outside there 
will be more negative electricity than positive and the 
atom will have a " negative charge" (27). 

The Electric Current. The attraction which an atom 
has for an electron varies greatly with the different 
species of atoms. The atoms of the so-called metals 
exert only a relatively weak attraction on electrons, 
whereas the attraction of the "non metals" appears to 
be greater. In a metal, therefore, it will be relatively 
easy to move electrons from place to place. 

When a stream of electrons is caused to move through 
the body of such a substance we have an electric current. 
From the modern point of view, therefore, an electric cur- 
rent in a wire is a stream of electrons moving through 
the relatively large spaces between the atoms or through 
the atoms themselves (28). 

The electrons forming the electric current move very 
slowly, perhaps only several inches a minute, but they 
move in enormous numbers. This speed must not be 
confused with the so-called " speed of electricity *' 
The far greater " speed of electricity" is due to the fact 
that the impulse is passed on very rapidly from electron 
to electron, so that when the electrons at the near end of 



a hundred mile wire are set moving those at the distant 
end are caused to take up the motion a very small frac- 
tion of a second later. Briefly, the actual speed of the 
electrons is very slow, but the rate of transmission of 
motion from electron to electron is very great. The 
action is closely similar to what follows when one end of 
a long rope is pulled. The impulse which results in the 
movement of the other end travels with much greater 
speed than the rope itself commonly attains. 

In the electrical case, the impulse to move travels with 
the speed of light, i.e., one hundred and eighty-six thou- 
sand miles a second, whereas the electrons themselves 
(i.e., the true electricity) move only a small fraction of 
an inch a minute. 

The Action of a Battery or Dynamo. Since it appears 
probable that electricity in its movable state always con- 
sists of one or more electrons, it is clear that no machine 
or device of any kind can produce electricity (29). What it 
does is to drive electricity. Hence a battery might be 
called "an electricity pump" or perhaps even "an elec- 
tron pump." Because of the chemical action taking place 
within the battery it is enabled to force electrons out 
through its negative terminal, and these electrons flow 
through the wires of the outside circuit and re-enter the 
battery again through the positive terminal. 

We pay a lighting company, therefore, not for "elec- 
tricity" but for electrical energy. Nor is electricity 
"used up" when the current passes through the fila- 
ment of an incandescent lamp. Precisely as many elec- 
trons leave the filament as enter it, but the stream as it 
passes through tends to set the atoms hi more violent 
vibration, and so heats or maintains the temperature of 
the filament. 

The electric transmission of power is thus closely 


analogous to the transmission of power by compressed 
air. We must stipulate, to improve the analogy, that the 
compressed air when "used" at the far end of the pipe- 
line be not set free, but returned by another pipe to the 
air compressor. Under these conditions, if the action 
goes on for a long enough time, the same air will go 
several times around the circuit. Ah* is not consumed, 
nor is it manufactured. It is simply compressed, that is, 
pumped around the circuit. The " consumer" who pays 
for compressed air under these circumstances gets value 
because the air comes to him at a high pressure and he 
sends it back at a low pressure. He has consumed 
energy and not air. 

The difference in ah* pressure in the pipes corresponds 
to the " voltage" of an electric transmission line (30). 

" Free Electrons" It has been said that the atoms of 
different elements seem to have different attractions for 
electrons. Recent experiments have made it probable 
that the so-called "positive" elements, including the 
metals, have a relatively weak attraction and the negative 
elements, such as sulphur, a powerful one. Many elec- 
trical phenomena probably owe their existence to this 

It is doubtless owing to this difference, for example, 
that metals conduct electricity so readily, whereas sub- 
stances like sulphur do not. We must suppose that the 
atoms of a metal have such a weak attraction for electrons 
that a vast number of the latter are in a practically free 
state throughout the body of the metal and are thus 
capable of being moved readily by any outside electric 
forces. This ease of movement makes the substance a 
good conductor. Atoms of such elements as sulphur, on 
the other hand, possess such great attraction for elec- 
trons that most of them are held tight in the atoms and 



cannot be moved easily from one place to another within 
the substance. This makes the material a "poor con- 
ductor," or, as we say, a "good insulator." 

There is good reason for believing that the electrons 
within a conductor act as regards heat motion as if they 
were small atoms, that is, they take part with the atoms or 
molecules hi the random vibration which appears to con- 
stitute the heat of a body. 

The "Evaporation" of Electrons. If electrons exist in 
large numbers within the substance of a metal it might 
be expected that if a metal were heated hot enough some 
of these would be given off into the surrounding space, 
after the manner in which a liquid loses molecules by 
evaporation. In fact this is found by experiment to be the 
case. The emission of electrons appears to be in every 
way analogous to the evaporation of a liquid (31). 


Electrons and Chemical Action. It seems probable 
that the forces involved in chemical affinity are electrical 
in character, that is, the atoms which form the groups 
known as molecules are held together by electric attrac- 
tion. Thus a molecule of hydrochloric acid is composed 
of one atom of hydrogen and one atom of chlorine, and 
the two cling together, probably because the chlorine atom 
has a negative charge, while the hydrogen atom has a 
positive one, and "unlike charges attract each other." 

When hydrogen gas and chlorine gas are put together 
in a vessel, heat and even light will cause them to combine 
suddenly and to form hydrochloric acid. That is, each 
atom of one kind becomes attached to one of the other 



[Chap. VI 

Fig. 10 

kind, forming a molecule of the new "compound," 
hydrochloric acid. 

We are on rather treacherous ground at this point, but 
we shall probably be not far wrong if we picture the 

mechanism of the process of 
union somewhat as follows. 
The light or heat detaches 
from some of the atoms a few 
electrons and these bound 
about at random between the 
molecules of the two sepa- 
rate gases. A very important 
fact then makes itself felt. 
As was said in the last chap- 
ter, different kinds of atoms 
have very different attrac- 
tions for electrons, and, hi 
the present case, the attrac- 
tion of the chlorine atoms is 
vastly greater than that of 
the hydrogen. Thus it will happen before long, since 
a few new electrons are being detached constantly, that 
every chlorine atom has one electron too many while every 
hydrogen atom has one electron too few. This means, of 
course, that each of the former attains a negative charge, 
and each of the latter a positive one. The remainder of 
the process consists in the attraction, and permanent 
combination in twos, of these atoms of unlike charge to 
form the groups which we call hydrochloric acid mole- 
cules (32), (33), (34). 

This theory of chemical action is not certain as yet 
but is worth mentioning. 

It is to be noticed that some kind of disturbance, in 
the above case heat or light, is necessary to keep up the 



The molecule which is symbolically 
represented above is one of hydro- 
chloric (muriatic) acid. As shown, 
it is made up of one atom of hydro- 
gen, //, combined with one atom 
of chlorine, CL The former bears 
a positive electrical charge, and the 
latter an equal negative charge. It 
is the attraction between these op- 
posite charges which is supposed to 
hold the molecule together. When 
the charged atoms are separated, 
as in "electrolytic dissociation" (see 
text), they form hydrogen and chlo- 
rine "ions." 


supply of "free electrons." We see, therefore, that 
were there no heat or light, or were the intensity of 
these below a certain limit, depending on the nature of 
the substances, we could get no chemical action (35). 
This inertness would probably be a property of all sub- 
stances in the dark at the so-called absolute zero of 
temperature (36). 

Atoms, Electrons and Light. There is good reason for 
believing that light-waves are electrical hi character. 
There seems to be no fundamental difference between 
light-waves and the electric waves used in wireless teleg- 
raphy, except that the latter are very much "longer" 
and the vibration is very much slower than in the former 
case. Light-waves and "wireless" waves are thus re- 
lated in the same way that a high-pitched sound is 
related to one of low pitch. 

"Wireless" waves (i.e., "Hertz" waves) are always 
produced by causing a charge of electricity to oscillate to 
and fro. According to the prevailing view, waves are 
thus set up hi the "ether of space " in a manner somewhat 
similar to the way sound-waves are set up by a vibrating 
bell (37). 

Since an oscillating charge is thus the cause of these 
waves it seems reasonable to ask what electric charge is 
responsible for the closely similar but vastly more rapid 
waves of light. This question has been asked, and an- 
swered by studying the effect of a powerful magnet on 
various sources of the vibrating charges within the glow- 
ing body which must be held responsible for the light- 
waves emitted. Results obtained by a distinguished 
Dutch scientist lead to the conclusion that the charge is 
that of the electron. Here again, therefore, we are thrown 
back on the same fundamental entity (38). 

The "radiant heat" from the sun is also of this electric- 


wave type of vibration so that the sun must be considered 
a light and heat radiator because of the vast number of 
vibrating electrons which it contains. 

To sum up: according to the modern wave theory, whenever 
an electric charge, whether this charge be that of one electron 
or many, vibrates back and forth, it radiates electrical waves 
which go out in all directions in space (39). If the oscil- 
lation is very slow they are called Hertzian waves, or 
"wireless" waves. If the oscillation is more rapid they 
are in general termed " heat-waves," and if the vibration 
is still more rapid the waves are capable of affecting the 
retina of the eye, and are called " light-waves " (40). 

All of these waves can be absorbed, refracted and 
reflected. They all transmit energy and hence are 
capable of heating any body which absorbs them. 

The Absorption of Electric Waves. The mechanism 
of the absorption of electric undulations is to be thought of 
as follows. As a sound-wave which is emitted from a 
vibrating body tends to set any body which it strikes into 
a similar vibration, so an electric wave which is emitted 
by an oscillating electric charge tends to set vibrating the 
electric charges within the body which it strikes. If 
these charges are so conditioned that they are capable of 
responding easily to the particular rapidity of vibration 
which thus strikes them, they will be set into violent 
vibration at the expense of the energy of the entering 
wave. This vibration ultimately becomes the random 
motion of heat. 

Since absorption is apparently due to motion of the 
electrons which a body contains, it follows that opaque- 
ness in bodies must be ascribed to this electron mobility. 
It is probable that if the electric charges within a body 
could be held fixed, the latter would be transparent to any 
electric wave. 



The Reflection of Electric Wanes. When the electrons 
within a body are set into vibration by an impinging 
light-wave they will, of course, act as radiators of new 
light-waves, and if the body has a flat surface those 
emitted from the electrons near the surface will join to 
form a definite single wave which travels back in an 
opposite direction to that of the entering one. This 
constitutes the reflected wave, which exists in general 
whenever light strikes a flat surface. The reflected 
wave may be almost as strong as that entering or it 
may be very weak, but except hi ideal cases it always 

Whether the surface is flat or not there will always be 
reflection, but if the surface has a certain degree of flat- 
ness the reflection will not be "diffuse" like that from a 
white-washed wall, but will be regular, like that from a 
mirror (41). 

The Speed of Electric Waves in Different Bodies. It 
is a generally accepted fact that all electric waves whether 
they correspond to light, heat or Hertzian waves, travel 
with the same speed in empty space, but with different 
speeds in material bodies. The explanation of this is 
that all material bodies contain electrical charges and that 
the wave which passes through the body is a complex 
resultant of the original entering wave and the secondary 
waves which are set up when the charges oscillate in 
response to the entering wave. This complex resulting 
wave, although it has the same vibration frequency as 
the original one, is altogether differently conditioned and 
it can be shown mathematically that it will not travel with 
the same speed (42). 




The Connection of Electricity with Magnetism. It is 
a well-known fact that magnetism always exists in the 
region surrounding a wire carrying an electric current. 
Since the trend of modern theory is towards the conclu- 
sion that an electric current always consists in the bodily 
movement of electrons or atoms, we must suppose mag- 
netism to accompany invariably the motion of electric 
charges, and indeed this has been found to be true (43). 

Any magnetic effect can best be magnified by winding 
the wire which is to carry the electric current around a 
piece of iron in the same general way that thread is 
wound upon a spool. The wire must, of course, be in- 
sulated so that the various turns do not touch each other. 
Such an iron spool may become a strong magnet when 
a current of electricity is sent through the wire surround- 
ing it. This is the principle at the basis of the action of 
the ordinary electric motor. The electric car is kept 
moving because the current which enters through the 
trolley wire, and leaves through the track, passes through 
a wire wound on a similar spool, and the spool (then a 
magnet) through its attraction sets rotating other pieces 
of iron which are attached to the wheels of the car. 

Deflection of Electrons Caused by Magnetism. It can be 
shown that if an electric charge is caused to move rapidly 
past the end of a magnet, the charge tends to be de- 
flected sidewise. This lateral deflection of electrons in 
a so-called " magnetic field" corresponds to one of the 
fundamental relations between electricity and magnetism, 



and by its means we have an important method of " gen- 
erating" an electric current, i.e., of moving electrons. 
Suppose for instance that a piece of wire is moved rapidly 
through a magnetic field, that is, near a magnet. The 
piece of wire in common with all conductors contains 
countless easily moved electrons, and when the wire is 
moved through the field the deflecting force mentioned 
above causes them to be pushed towards one end of the 
wire, If the latter is held in the proper position (44). 

The Action of a Dynamo. The ordinary dynamo or 
electric generator, as it is often called, is a machine for 
moving wires rapidly through a magnetic field, and for 
collecting the electric current which is set up by the de- 
flecting forces above mentioned. Such a machine has, 
of course, to be run by some outside source of power such 
as a steam-engine, or a water-wheel. 

Permanent Magnetism. The familiar type of magnet 
which is not maintained by means of an electric current, 
the small red-painted horse-shoe magnet of the toy-shops, 
for example, is called technically a " permanent magnet." 
It is probable that the ultimate cause of magnetism in 
this case is quite similar to that in the case of the spool 
before mentioned. In the latter, electrons circulate in a 
helix around the outside of a large piece of metal, and in 
the former, although there is no circulation of electrons 
around any visible path, helical or circular, still such 
paths probably exist within the molecules of the iron which 
forms the permanent magnet. According to the present 
belief, the electrons within a piece of magnetized iron do 
not move in a wholly random fashion, but have what we 
might call a " helical prejudice," and perhaps even re- 
volve in circles within the molecules themselves. 

It is now generally believed that the cause of magnetism 
is always the motion of electrical charges (45). 



The Effect of Magnetism on Light. Since the source 
of the light which an incandescent body emits lies in the 
vibratory motions of electrons within the body, and since 
magnetism has a tendency to deflect moving electrons, 
we should not be surprised to find that certain sources of 
light when put near a powerful magnet have their emitted 
light modified in a complicated way. This has been found 
to be the case, and the results tend to verify the views 
herein set forth (38). 


There is a remarkable group of substances which emit 
rays continuously, without obtaining energy from their 
surroundings. Radium is, perhaps, the most striking 
member of this group and it is now known with consider- 
able surety that in its case the rays would only fall off to 
about half then* present intensity in two thousand years. 
This extraordinary action goes under the general name of 
" radio-activity" and the substances which emit the rays 
are known as "radio-active substances" (46). 

The Three Rays. The rays emitted are not all of 
the same kind, but are composed of three distinct types, 
each with definite characteristics of its own. It was 
thought convenient in the beginning to designate these 
rays by the first three of the Greek letters, and they are 
therefore known as the alpha rays, the beta rays, and the 
gamma rays. 

The Beta Rays. There is now no reasonable doubt 
that the beta rays consist of a stream of electrons com- 
ing out of the substance like bullets from a machine gun. 
The most surprising thing about these rays is the enor- 



mous velocity of the electrons. Their speed is almost 
equal to that of light, or about one hundred and eighty- 
six thousand miles a second. This inconceivable velocity 
is measured indirectly, but by methods which make it 
almost certain that the result is correct. This speed 
enables part of the rays to penetrate a plate of metal as 
thick as an ordinary book cover (47). 

The Alpha Rays. Within the last few years it has 
become practically certain that these rays consist of a 
stream of atoms of the element helium. This surprising 
fact could certainly not have been predicted when first 
the rays were discovered. The helium atoms have each 
a double positive charge ; each atom, that is, has lost two 
electrons. The atoms of the stream have a velocity of 
about one-tenth that of light, or about eighteen thou- 
sand miles per second. This is less than the speed of 
the beta rays, but it must be remembered that the atom 
of helium, whose " atomic weight" is four, is about eight 
thousand times as heavy as an electron, and thus, al- 
though the helium atoms of the alpha rays are moving 
more slowly, their energy is much greater than that of 
the beta ray particles, the electrons (48). 

This energy is in fact so great that a single "alpha 
particle," that is, a single atom of helium, when it strikes 
certain substances, will cause them to phosphoresce 
(or more properly fluoresce) for an instant over a region 
large enough to be seen with a lens. This makes visible 
the effect due to a single atom and gives us the first case 
in the history of science where any effect caused by one atom 
has been observed. 

The Gamma Rays. There has been some dispute as 
to the ultimate nature of the gamma rays, but it is now 
practically certain that they do not consist of particles in 
the ordinary sense, but of impulses similar to those con- 



[Chap. VHI 


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Mj0-ftww*f2 A o 

6.9 Ao<"* 




#A0/o-7#ofr/</Af of) 


+ OC. 


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THOW(/MM. ae /goV a 

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Fig. lla 


These diagrams represent symbolically the four series of radio-active substances. 
In each vertical line the elements above break down to produce those immediately 
below, as indicated by the arrows. During this decomposition they emit the types 
of radiation designated by the Greek characters attached to the horizontal arrows. 
Some of the substances, such as Meso-Thorium 1 and Actinium, are rayless. The 
periods of time indicated beside each element stand for the intervals required in 
order that half of a given quantity of the element in question should have decom- 
posed. Thus, if we set aside a gram of Radium to-day it will amount to only half a 
gram when two thousand years have elapsed. The other half will have gone 
through the change symbolized in the diagram. The figures inside the circles are 
the respective atomic weights ; those outside, the atomic numbers. 



25^*25 **<Su* 

26.S Jays 


0. 00 J second 

L/&AN/UM 2. 

/OA//L/M nx, ^^LKV 
Z00,000years " ^0^ Ot 


Jd. 5 minutes 


Fig. lib 


stituting the X Rays and not very dissimilar to the waves 
or impulses making up light. 

The connection between the beta rays and the gamma 
rays is probably similar to that between the bullet and the 
sound in the case of a gun. Both the bullet and the sound 
of the explosion travel away from the mouth of the gun, 
but the bullet is a moving object while the sound is a dis- 
turbance, or a wave impulse in the air. In perhaps the 
same general way both the beta rays and the gamma 
rays come out of the radio-active substance, but the 
beta particle is a moving electron while the gamma 
rays are probably electric wave impulses. Both rays, 
however, like the bullet and sound, leave the source 
together (49). 

Cause of Radio- Activity. It is now generally believed 
that the cause of radio-activity is the actual breaking up 
of the atoms of the radiating substance. The radium 
atom " explodes," like a bomb, into two "pieces" and 
the energy of the explosion gives each "piece" a high 
velocity. The two pieces are a helium atom and an atom 
of an element previously unknown. The new element 
is called "niton" or "radium emanation" and appears 
to be a heavy gas belonging to the same group in the 
periodic system of elements as do helium and argon. 

We have then, in radio-activity, a case of natural 
alchemy, but since it has been found impossible, so far, 
to hasten or retard the process artificially, the knowledge 
of radio-activity would not have helped the alchemists in 
their quest. 

Successive Disruptions of Radio-Active Atoms. The 
radium atom appears, in fact, to be a member of a chain 
of "explosive atoms" for the atom of "niton" itself is 
radio-active. If given time, it will itself "explode" and 
give off another helium atom. The chain has been fol- 



lowed out through nearly a dozen elements, as shown in 
Figure 11. 

Radio-Activity Not a Chemical Change. It must be 
borne in mind that the change accompanying the process 
of radio-activity is not a chemical change. As has already 
been stated, a chemical change involves the regrouping of 
the atoms to form new molecules. This does not affect 
the integrity of the individual atoms but only their rela- 
tions to then* neighbors. In radio-activity, on the other 
hand, we have changes which involve the actual disrup- 
tion of the atom. 

Intra-Atomic Energy. When coal is burned, the energy 
is derived from the attraction existing between the atoms 
of carbon (coal) and the oxygen atoms of the air, and it 
should be evident that hi general whenever there are 
strong attractions or repulsions between different atoms, 
there is a possibility of obtaining energy by allowing these 
forces to act. The energy developed in radio-activity 
has also its origin in forces acting within the substance, 
but they are not the forces acting between the atoms, as is 
the case with all chemical energy, but those acting within 
the atom, the so-called " intra-atomic forces." 

We may therefore say with truth that all the energy 
obtained in the world from fuel is of the extra-atomic 
variety, while the energy of radio-activity is something 
quite new. It comes out of the atom and is therefore 
intra-atomic energy. 

The Quantity of Intra-Atomic Energy. One of the 
most remarkable facts about these intra-atomic forces is 
then* great size compared with the ex/ra-atomic forces 
of chemical action. The slowness of the radio-active 
changes tends to conceal this but it is nevertheless 

The energy given off by radium ultimately becomes 



heat and this heat has been directly measured a number 
of times. Since the radium would be hah* gone in two 
thousand years, a simple calculation shows that the 
total energy given up by radium and its products during 
transformation is about a quarter of a million times the 
energy to be obtained by burning an equal weight of coal. To 
make this statement a little more vivid, the fact might be 
noted as an illustration that a large ocean steamship 
could make one ocean passage on the energy from a few 
pounds of radium and its products (50). 

The Radio-Active Elements. There appears to be noth- 
ing unusual about the radio-active elements except their 
radio-activity. For example, referring to Figure 11, 
radium itself belongs to the same chemical family as 
barium and behaves chemically like barium. The next 
element, the atoms of which are formed from the dis- 
ruption of the radium atom is the gas called " niton" 
(radium emanation) already mentioned. The following 
one, " radium A," so called, is a solid substance. 

The atomic weight of radium is about 226, that of 
niton about 222 and that of "radium A" about 218. 
These numbers correspond with the fact that the alpha 
particle (the helium atom) which comes off at each ex- 
plosion has an atomic weight of 4, so that each of the 
above elements has a weight of 4 less than the element 
from which it sprung. 

Are All of the Elements Radio- Active? There is some 
reason to believe, although as yet no proof, that all ele- 
ments are decomposing in the same way, or in other words, 
that radio-activity is a universal property of matter. This 
means, of course, that in the case of the well-known 
" permanent" elements the process is so slow as not to 
be noticeable. Already potassium has shown signs of 
being radio-active and as methods of measurement be- 



come gradually more delicate it is not improbable that 
other elements will do the same (51). 

The Evolution of the Elements. The probability that 
all elements are to some degree radio-active has led to 
the general conception of what is called the "evolution 
of the elements." According to this view there is a slow 
forming of the lighter elements through the disintegra- 
tion of the heavier ones. The well-known elements are 
to be considered as merely those whose average "life" 
is so great as to make them, from a human point of view, 
permanent. If this conception be a true one, if the atoms 
of any element are descendants, as it were, of the atoms 
of heavier elements, then it is not difficult to understand 
in a general way why the well-known relations of the 
periodic system exist between the elements. 

One objection which has been raised to this view is 
that, did such evolution exist, it must have completed 
itself years ago and hence only the lighter elements 
should now remain. There are, however, some faint 
indications that under conditions differing greatly from 
those on the earth, such namely as exist in the hotter 
stars, the reverse process may be going on. This would 
involve the building up of the heavier elements out of 
the lighter ones. At present, however, such a sugges- 
tion is mere speculation (52). 


The study of radio-activity opened the door to the 
knowledge of intra-atomic phenomena and recently it 
has given us some definite ideas as to the details of 
atomic structure. 



General Principles. One or two general statements 
must be made at the outset. In the first place, electricity 
certainly plays a highly important role in the formation 
of the atom and the intra-atomic forces must be largely 
electrical. Whether all intra-atomic force is of this nature 
is a question which cannot be answered at present. 

Secondly, we can say without much chance of ultimate 
contradiction that the atom is very much more "porous," 
or "open work," in structure than the appearance of the 
vast aggregates of atoms which we know as "bodies," 
would lead us to believe. It has been stated that some of 
the beta particles (electrons) from radium, travelling with 
nearly the velocity of light, go straight through a piece of 
metal as thick as the average book cover, and it needs 
but a very short calculation to show that this means that 
the electron penetrates more than a million atoms one 
after the other. It must be remembered that in a solid 
substance the atoms are quite near together so that such 
a flying electron has no chance of travelling entirely in 
the space between the atoms during its whole passage 
through the metal. 

If the atom is made up almost entirely of electrons, it is 
easy to show that, on the average, the electrons must be 
as far apart compared with their size as are the planets 
in the solar system. This at first sight seems incredible, 
but it is not more so than many of the accepted facts of 

The Evidence for Orderly Structure in the Atom; Spectral 
Lines. Although much of the light and heat given out 
by bodies seems to be due to the random vibration of the 
electrons within them, this is by no means invariably the 
case. Almost all gases when caused to give out light, emit 
it in the form of "pure notes," that is in the form of waves 
of definite frequency. The light which we get from a 



white-hot nail consists of all possible frequencies of vibra- 
tion. It is similar to what we should obtain in sound if 
we took a long rod of wood and struck all the keys of a 
piano at once. The light, however, which we get from a 
glowing gas, such light as comes from the long glass tubes 
now frequently used in garages and known as Cooper 
Hewitt Lamps, is not of this chaotic composition. It can 
be shown to consist of " a few pure notes," to use the same 
acoustical analogy. These pure " notes," or colors, are 
superimposed upon each other and hence the combina- 
tion is a close analogy to what we call in acoustics a 
musical chord. 

The definite "light notes" emitted by one substance 
when in the gaseous form are perfectly characteristic of 
that particular species of atom and no other kind of atom 
gives of the same notes; no other kind of atom, that is, 
emits light of just the same frequencies. 

This can only mean that the electrons within, or about, 
one kind of atom are in some definite arrangement or 
move in some definite way and one which differs mark- 
edly from that to be found in atoms of other species (53). 



The last five years have brought marked developments 
in fundamental physical theory. 

Of the three fundamental entities, atom, electron and 
radiant energy, the electron alone has remained to our 
view about where it was a half decade ago, but our con- 
ceptions of the atom and of radiant energy have changed 



Recent Advances Concerning the Atom. New light on 
the structure of the atom has come largely through the 
study of radio-active substances. From researches of 
Rutherford and his associates, it now seems highly prob- 
able that the atom is made up of a minute, positively 
charged nucleus surrounded by several rings or, better, 
regions of electrons. The total number of electrons is 
such that their total negative charge is equal to the posi- 
tive charge on the nucleus. 

Since the volume of the nucleus and the volume of the 
electrons is in all cases very small compared with the di- 
mensions of the system, the major part of the volume of 
an atom is unoccupied in the ordinary sense of the word. 
This space is " empty" in the same sense that the space 
around a large charged body is empty, and the law of 
electric attraction and repulsion within the atom appears 
to be the same as for large charges; namely: the well- 
known " inverse square law." 

What are at present believed to be the facts regarding 
atomic structure are, perhaps, somewhat intricate, and 
clearness can best be attained by making a formal list. 

1. The atom consists of nucleus and surrounding electrons. 

2. The electrons are negatively charged; the nucleus posi- 
tively charged. 

3. Both electrons and nucleus are very small compared to the 
distances between them. 

4. The nucleus is composed of a certain amount of negative 
electricity and a larger total amount of positive electricity. 

5. The nucleus may lose either negative or positive electric- 
ity, but it always does so in definite units, the negative unit being 
the electron and the positive unit equal to two electrons. Such 
losses occur only during radio-activity. 1 

6. The law of electric attraction between the nucleus and elec- 
trons in the surrounding space is the familiar "inverse square law." 

1 Nothing is known as yet regarding actions or relations inside 
the nucleus. 



7. The surrounding electrons have a total charge equal to the 
positive charge on the nucleus, so that an atom in the ordinary 
state has no resultant charge. 

8. The electrons surrounding the nucleus exist in two or more 
rings or regions at markedly different distances from the nucleus. 

9. The more distant electrons are those chiefly concerned in 
chemical action. 

10. The chemical properties of an element are determined chiefly 
or entirely by the number of electrons surrounding the nucleus 
and hence by the charge on the nucleus. (See 7 above.) 

11. If, therefore, a nucleus loses two negative units (electrons) 
and one positive unit (see 5) the atom which results will not be 
distinguishable chemically from the original one. 

12. The above action will, however, result in a loss of weight 
and the new atom will, therefore, have a less atomic weight than 
the original, although chemists will have given the two elements 
the same name. 1 

13. The electrons surrounding the nucleus are probably in 
motion about it. 

14. Violent physical phenomena may temporarily remove elec- 
trons from the atom (but not from the nucleus) and, in chemical 
combination, one atom may have some of the electrons belonging 
to another. 

15. During the change in grouping or during the removal of 
electrons from the region surrounding the nucleus, electric waves 
(light, X Rays) are radiated. 

16. Changes in the nucleus only occur during the process of 
radio-activity and cannot therefore in any way be caused to happen 

17. The " beta rays" are electrons escaping from the nucleus 
and the " alpha rays" are positive units escaping from it. 

18. The "gamma rays" are the electric wave radiations result- 
ing when electrons are violently ejected from the nucleus during 

19. There is some mysterious action at work which prevents 
the surrounding electrons from falling into the nucleus. This 
action is probably closely related with the existence of the " Quanta " 
of Planck (vide infra). 

1 Two such chemically identical elements with different atomic 
weights are now called " isotopes." 



Atomic Numbers. Since the chemical properties of 
an atom seem to be determined principally, if not entirely, 
by the resultant charge of the nucleus, it is this charge 
which fixes the place of the atom in the " Periodic Table," 
so familiar to chemists (see Section 6). A series of num- 
bers have therefore been defined, and called the " atomic 
numbers," which appear to be more vital to the chemist 
than the atomic weight. The reason for this, as men- 
tioned before (see 11 and 12, above), is that the weight 
of the nucleus depends on its internal make-up, while 
its action on the surrounding electrons depends only 
on its resultant charge, that is, only on the excess of 
positive over negative electricity present. Weight is not 
an interesting chemical feature ; resultant charge (meas- 
ured by the atomic number) is. 

The "Quantum" Theory. A decade ago an epoch- 
making paper was published by Max Planck, Professor 
of Physics in the University of Berlin. The revolutionary 
feature in Planck's work was the suggestion that the 
atoms of matter did not radiate energy continuously, but 
in small definite units. In the more radical forms of 
the theory, as developed by other investigators, radiant 
energy, such as energy from the sun, is considered to 
be in the form of separate units in space: "bullets of 
energy," one might say. This means that the light and 
heat from the sun fall on the earth like a shower of rain. 
Planck called the separate units " Light Quantities." l 

This conception of radiation is in marked contrast to 
the classical wave-motion theory which unquestionably 
represents a vast host of facts almost ideally. Accord- 
ing to the wave-motion view, a disturbed electron radiates 
a continuous wave in all directions like a stone dropped 
in a pond. If the electron made a regular series of vibra- 
* "Licht Quanta." 


tions, a train of such waves would spread out in concentric 
spheres and we should have continuous light radiation. 

In Planck's theory, however, it is more as if the electron 
emitted bullets of radiation. Just how this occurs he does 
not attempt to say. 

Of course a mere baseless suggestion is worth nothing 
in physics and Planck would not have hinted at such a 
heterodox notion had he not found that a hitherto stub- 
born paradox in the theory of radiation was apparently 
removed by it. 

It will be remembered that all bodies by virtue of their 
temperature send out radiant energy. In general the 
higher the temperature the more total energy is radiated 
in a second and also the higher is the average frequency 
(39) of the radiation. In the case of light this is equivalent 
to saying that the higher the temperature the brighter 
and the bluer is the emitted light. 

Now with certain limitations the amount of this radia- 
tion had been found by experiment, but up to the time of 
Planck's work no one had been able to give a satisfactory 
theoretical explanation of the results. The two most 
notable attempts at explanation were those of Raylei^h 
and Wien. Each of these covered satisfactorily part of 
the facts but neither covered all. Planck found that by 
using the new hypothesis he could not only give an ex- 
planation of the experimental results, but his mathemat- 
ical formula contained within it both the Rayleigh and 
the Wien formulae. 

It was this success which gave Planck the courage to 
make such an extremely heterodox suggestion as that of 
light radiation in unit.:;. 

Although meeting with this initial success it has only 
been within the last few years that Planck's view has, in 
the minds of those most competent to judge, passed from 



a possibility to a probability. This has been due not only 
to further study of the direct problem of radiation, but to 
indirect evidence received from other fields of physics. 
Perhaps the most important confirmation of the new 
view has come from a study of the action of light on 
metals. Certain experiments along this line, notably 
those of Professor Millikan of the University of Chicago, 
give results which in the opinion of some are almost con- 
clusive evidence for the existence of "Quanta." 

Less direct, but also pointing hi the same direction are 
the results of recent very low temperature research. 

The " Quantum Hypothesis" has certainly become a 
very impressive one, to say the least. Just how impress- 
ive, will of course depend hi part on the temperament 
and a priori ideas of the investigator. 

As to a priori probability it should be noticed that at 
one time matter and electricity were both pictured as 
continuous but that further knowledge showed that they 
were both atomic. It does not seem very strange that 
the third member of the trio of physical fundamentals, 
namely radiant energy, should also turn out to be 

The real difficulty before the Quantum Hypothesis is 
the necessity of joining hands with the classical electrical 
theory. There can at present be no reasonable doubt 
that the oscillation of large charges, as in wireless teleg- 
raphy, sends out long waves in the manner required by 
classical theory. Observation and experiment are unani- 
mous in their approval of the well-known electrical laws. 
It is only when we get to the minute charges in a single 
atom that new laws seem to make their appearance. 

Perhaps the most important question which the theory 
of radiation presents to-day is therefore the following. 
How is it possible for big slow phenomena to follow 



one set of laws and for small quick phenomena which 
are unquestionably of the same general nature, to follow 
another set? 

The answer to this cannot now be given, but it seems 
probable that the answer when it comes will in some way 
show that one set of laws corresponds to the " averaging 
out" of the other set. The way in which an army moves, 
as seen from a high balloon, is quite different from the 
way a single man moves, and yet it is made up of men. 
The army moves continuously, but a single man moves 
jerkily in steps. 

The Quantum Hypothesis is at the very center of dis- 
cussion and controversy in present-day physics. 

The Similarity of All Forms of Radiant Energy; X 
Rays. Until very recently it had never been found 
possible to reflect X rays in the way in which light is so 
easily reflected from a mirror. 

It had been suggested by several investigators that this 
was probably due to the fact that X rays were waves of 
such extremely short length that the flattest surface that 
could be made was too rough to reflect them. If any 
surface, even a polished one, be examined with a high- 
power microscope, it will be found to be rough and ir- 
regular in detail, and the idea was that such a surface, 
although flat for light waves, might be like a ploughed 
field for the extremely more minute X rays. 

The obtaining of a theoretically " smooth" surface for 
X rays appeared hopeless, therefore, until Laue sug- 
gested in 1910 that natural crystals, if carefully broken, 
should present a surface smooth and flat beyond any- 
thing artificially possible. This would be due to the fact 
that, hi a freshly broken crystal surface, the molecules 
would be arranged regularly, like the bricks in a pave- 



As a matter of fact, when a crystal of mica was so broken 
and tested, beautiful X ray reflection resulted. This 
simple, but epoch-making experiment, served as the 
starting point of a long series of experiments by various 
investigators which have resulted in practically proving 
that X rays are precisely similar to light, but have a 
shorter wave-length. 

Just as X rays may now be looked upon as light waves 
of extremely short wave-length, so the gamma rays which 
come from radio-active substances may be regarded as X 
rays of exceedingly short wave-length. At the present 
time, therefore, it is very probable that all forms of 
radiation, from Hertz waves through "heat," light, ultra- 
violet light, and X rays, to gamma rays are essentially 
similar in form, though how they can at once show wave 
characteristics and the bullet characteristics of the Quan- 
tum view is still a puzzle. 


To avoid possible objections to the general validity of 
the modern theory of matter it may be well to point out 
the relation of atomic ideas to the physical structure of 
living things. Biologists say that all living things are 
composed of cells, minute objects which are visible in 
the microscope and have a definite structure. Typically, 
then* shape is not very distant from that of a sphere, and 
the so-called "cell-wall," like the skin of an orange, 
simply surrounds the whole. These cells, they say, are 
singularly independent, sometimes even capable of living 
alone, in which case the single cell is called a "one-celled 
animal" (or plant). 



From a modern point of view the cell is thus the "vital 
unit" of all living things. It performs for itself all the 
life functions which the vast aggregates of cells which we 
know as animals or plants also do. 

Now it is clear that if we are to consider all matter to 
have atomic structure, living substance as well as "dead," 
the atom must be very small indeed compared with the 
cell, for otherwise there would be too few possibilities of 
structure to account for the wonderful complexity of cell 
action. A rich mosaic cannot be made from a small 
number of pieces. 

The complexity of cell action and that of living things 
in general does not, however, in any way contradict the 
ideas of atomic structure here outlined, for the reason 
that most cells are so large as to be seen clearly with the 
microscope. Thus the smallest cell so far studied with 
the microscope certainly contains several million atoms 
at least, and hence the "mosaic possibilities" are almost 

From a fundamental point of view we must look upon 
the physical side of the changes known as life processes 
as consisting in the continual rearrangement, grouping 
and regrouping, of the molecules themselves or of the 
atoms making up the molecules. With so many atoms 
composing the physical substance of a single cell the 
possibilities of change are vast (55). 




Section 1 


The foundation of the "new physics" lies in the work 
of Michael Faraday upon electro-magnetism and the con- 
duction of electricity in solutions, as well as in the sub- 
sequent development of his ideas by J. Clerk Maxwell. 
Its corner-stone was laid by J. J. Thomson when, hi 1898, 
he discovered the electron. The investigations of Maxwell 
had shown that light is probably an electrical wave-motion, 
and this conception was strongly confirmed by the dis- 
covery of the long electrical waves by H. R. Hertz in 1887. 
The idea of "a molecule of electricity" was clearly sug- 
gested by Maxwell, and the name " electron" was ap- 
plied to the conception as early as 1881 by G. J. Stoney. 
The definite physical demonstration of its reality dates 
from the more recent work of Thomson, P. A. Lenard, 
H. A. Lorentz, and others, who have shown that electrons 
in then* vibrations undoubtedly generate light. 

Another line of discovery which has been of tremendous 
importance to the modern theory lies hi the field of radio- 
activity, which was opened by the investigations of Henri 
Becquerel in 1896. The conception of the atom as the 
indestructible basis of chemical change was first system- 
atically defended by John Dalton in 1808. The science of 
radio-activity has not only provided us with a new source 
of electrons, but it has led us to the conclusion that the 

NOTE : The Section numbers coincide with the full-face index 
numbers in the text of Part I. 



chemical atom, although a fundamental reality, is never- 
theless destructible. Of late years the most important 
work in this new science has been done by Ernest Ruther- 
ford, whose treatise on the subject is now standard. 

The discovery of "X Rays" by W. C. Roentgen in 
1895 was another important event in the development of 
the modern conception of the constitution of matter. 
The problem as to their exact nature has only recently 
been settled, but with its solution have come remarkable 
advances in our knowledge of the structure of crystalline 

During the past fifteen years a host of new workers have 
appeared and progress has been proportionately rapid. 
At the present time we seem to be on the verge of epoch- 
making discoveries with regard to the nature and laws of 
radiant energy (or light), which may bring about as funda- 
mental a change in the ideas of ultimate physics as did 
the discovery of the electron and of the phenomena which 
it underlies. Besides this the most recent develop- 
ment of all in the theory of "isotopism" and " atomic 
numbers" new light is being thrown on the fundamental 
mystery of the periodic table of the chemical elements. 


An ably written sketch of the history of electricity by J. A. Flem- 
ming is to be found under " Electricity" in the llth edition of the 
Encyclopaedia Britannica. The article "Atom," in the same work, 
may also be consulted to advantage in connection with the history 
of the theory of matter. 

Section 2 

There are a great many possible ways of determining 
the approximate size of atoms and molecules, but those 
which are the simplest and most direct are unfortunately 
the least accurate. 



Thin Films. It is obvious that if the atoms have a 
permanent shape and volume they must be at least as 
small in diameter as the thinnest known films of matter 
are thick. When gold is flattened by the process of gold- 
beating the atoms are spread out over a surface greater 
than that which they previously covered. Now a sheet 
of the thinnest gold-leaf measures about one one-mil- 
lionth of an inch in thickness so that the atoms of gold 
-which are among the largest known must have a 
diameter at least as small as one one-millionth of an inch. 
However, we are familiar with liquid films which are much 
thinner than gold-leaf. The walls of soap-bubbles and 
the films produced by pouring oil in very small quan- 
tities upon water are sometimes as thin as one twenty- 
millionth of an inch, and consequently the diameter of 
the atoms which compose them cannot be greater than 
this figure. 

Indirect Methods. To arrive at more exact estimates 
of the size of atoms it is necessary to resort to calcula- 
tion. Such a method of course involves some assump- 
tions, but there are many different ways of calculating 
atomic sizes in which nearly as many different assump- 
tions are employed, and as they all lead to substantially 
the same results they cannot be far from the truth. The 
facts upon which calculations of this sort have been based 
vary from the resistance offered to the passage of the 
electric current through metals and liquids, to the blue 
color of the sky. 

The Number of Atoms in a Given Volume. It is clear 
that if we were acquainted with the number of atoms 
contained in a given body we could specify an upper limit 
for the size of a single atom, by dividing the volume of the 
body as a whole by the number of atoms which it contained. 
Of course this calculation would not provide us with an 



exact measure of the atoms themselves, because all of the 
space in the body is not occupied by the atoms. This 
is due in part to the fact that the atoms are separated under 
the influence of the constant vibratory motion which goes 
on among them, but it would also be true even if they 
were entirely quiescent, which would be the case at the 
so-called "absolute zero" of temperature, as explained 
further on in Part I. However, the atoms alone could 
not be larger than the size determined by such a calcula- 
tion, and if the data upon which it was based were taken 
from a solid substance at or near absolute zero it would 
come very close to an accurate measure of the actual 
atomic size. 

Now it happens that it is relatively easy to discover 
the number of atoms which are contained in a given 
weight of almost any substance. One very simple and 
accurate method of doing this is to weigh the amount of 
the substance which is deposited out of solution by the 
passage of a certain quantity of electricity. Each atomic 
particle thus deposited carries with it a definite and known 
quantity of electricity, the natural unit of electricity or 
some simple multiple of this unit. 1 The number of times 
this unit is contained in the quantity of electricity which 
has passed gives us a measure of the number of atoms in 
the amount of substance which has been deposited by 
the current. Since this substance can be collected and 
its volume determined, we are able to specify the number 
of atoms contained in a given volume of the substance 
in question, although the problem is slightly more complex 
than we have represented it. 

From this we can calculate easily the upper limit of 
atomic size to which we have referred. 

1 Which happens to be the charge borne by the electron, and 
which can be determined by other means. 



Recently it has been found possible to count individu- 
ally the helium atoms (see Section 48), which are shot 
off from radio-active substances, so that after a measur- 
able amount of this gas has collected, the number of atoms 
(77 billion billion per cubic inch) of which it is made up 
is accurately known. 

Other Methods: the Atoms of Gases. Another fairly 
direct way of getting the magnitude of atoms depends upon 
a measurement of the speed at which atoms move through 
a liquid or a gas under the influence of a known force. 
The smaller the particles into which a given quantity of 
matter is divided the more slowly it will move through a 
fluid under the action of a fixed force. Everybody is 
familiar with this fact in the case of falling dust particles, 
which are all acted upon by one force, gravity. The larger 
ones settle very rapidly, whereas the small particles may 
remain apparently motionless in quiet air for a long time. 
Under the right conditions atoms can be made to move 
through liquids and gases under forces as definitely 
known as that of gravity, and although these substances 
are themselves made up of atoms the nature of the resist- 
ance which the moving atoms encounter is probably not 
very different from that which would be offered to the 
passage of larger bodies. Of course we cannot measure 
the rate of motion of single atoms, but we can determine 
that of large numbers of them, which is equally satisfac- 
tory to our purpose. Knowing the speed of the travel- 
ling atoms and the magnitude of the forces which are 
propelling them, we can easily calculate their diameters, 
which according to this method turn out to be about one 
one-hundred-millionth of an inch. 

Still more accurate methods are known for the calcu- 
lation of atomic sizes, but most of these are dependent 
upon complex considerations into which we cannot enter 



here. For example, the size of the atoms of a gas can be 
calculated from the rate at which heat is conducted 
through the body of the gas. Heat consists in the rapid 
random motion of the gas molecules, and it is clear that 
the larger these molecules are, the more they will impede 
each other's motion and, consequently, the more slowly 
this motion will be transferred from one part of the gas to 
another. But such a transfer is identical with the conduc- 
tion of heat. If we combine our knowledge of the rate of 
conduction with that of the number of molecules contained 
within a given volume of the gas we can calculate the size 
of the molecules themselves. In the case of simple sub- 
stances, molecular and atomic sizes are of the same 
order of magnitude. 

Other methods of figuring atomic sizes depend upon 
measurements of the so-called "viscosity" of gases, or 
their internal friction. It is also possible to get a very 
exact idea of the diameter of the gas molecules by study- 
ing the deviations from the well-known "law of Boyle" 
(see Section 18 below), which states that the volume of a 
gas is inversely proportional to the pressure under which 
it is confined. These deviations are due to the influence 
of the volume of the atoms themselves upon that of the 
gas as a whole. The atoms in most gases are very far 
apart compared with their sizes, but as the gas is com- 
pressed that is, as the atoms are forced nearer and 
nearer together the atomic volume begins to make 
itself felt, and this fact provides us with a basis upon 
which to calculate that volume. Another well-known 
method is founded upon the rates at which gases "dif- 
fuse" into each other, and modern studies in electricity 
have yielded other very exact means of determining the 
diameters of atoms. The new science of radio-activity 
has also contributed to our knowledge of atomic sizes by 



furnishing the experimenter with individual atoms bear- 
ing electrical charges to make them conspicuous, and 
moving at almost inconceivably high velocities. 

Agreement of Results. The results of these diverse 
methods of discovering the magnitude of atoms are in a 
harmony so substantial that it is practically impossible 
to doubt their truth. The diameter of the smallest atoms 
must be closely in the neighborhood of one three-hun- 
dred-millionth of an inch, and that of the largest cannot 
be many times this. What may perhaps be called "the 
realm of atomic magnitudes " is accordingly very definite 
and limited. 


For a more detailed discussion of the means by which the sizes 
of atoms and molecules are calculated the reader may consult: 

A. D. Risteen's "Molecules and the Molecular Theory," 1895, 
pp. 133-151. 

A list of specific molecular diameters, calculated from modern 
measurements is given by W. Sutherland in the English Philo- 
sophical Magazine for February, 1908, vol. 17, pp. 320-321. A very 
simple discussion of atomic sizes will be found in A. E. Dolbear's 
"Matter, Ether, and Motion," enlarged edition, 1894, pp. 8-26. 

On molecular volumes and their calculation, see W. Nernst's 
"Theoretical Chemistry," (Eng. Trans.), pp. 304-307. 

Section 3 

To make a single atom visible to the eye we should 
require a microscope capable of enlarging the apparent 
size of objects nearly a million times. The so-called 
" ultra-microscope" has a power closely approaching this, 
and still more powerful instruments might be built if 
optical principles did not interfere. Objects very much 
smaller than light waves will not reflect light in the usual 
way, /.c., so that their surfaces can be seen. However, 
they are often detectable under the ultra-microscope as 



minute luminous specks or points. Particles having a 
diameter of about three millionths of an inch have been 
detected by means of the ultra-microscope. This is 
approximately two hundred times the diameter of the 
average atom. Whether or not we shall ever be able to 
prove the existence of atoms and molecules by sight is a 
question which must be decided by the progress of opti- 
cal invention, but it is well to bear in mind the fact that 
even now the limits of visibility lie not very far from the 
realm of atomic sizes. 

The particles seen under the ultra-microscope are 
those characteristic of so-called colloidal substances. 
There is a continuous gradation of sizes among such 
particles, from those of strictly molecular magnitude up 
to the very much larger granules which are found in an 
"emulsion" or a "suspension." There is thus no 
sharp line dividing colloidal particles from large mole- 
cules but, nevertheless, colloidal substances have re- 
markable properties which distinguish them from strictly 
molecular, or "crystalloid" bodies. These properties 
are exhibited most distinctly in aqueous solution or sus- 
pension, and include such phenomena as coagulation 
and gelatination. Most of the substances characteristic 
of living organisms are in the colloidal state, and it is 
almost certain that life would be impossible without 
colloids. However, the fundamental peculiarity about 
colloids, is simply their degree of subdivision, or "dis- 
persion," as it is sometimes called. 


On the use and results of the ultra-microscope see R. Zsigmondy's 
"Colloids and the Ultra-Microscope" (Eng. Trans.), 1909, esp. 
page 122 on the limits of visibility. 

For a popular discussion of the questions above raised see A. E. 
Dolbear's " Matter, Ether, and Motion," enlarged edition, 1894, 
pp. 8-26. 



Section 4 

Why Atoms are Supposed to be Spherical. It is obvious 
that the shape of an atom or molecule will materially 
affect certain of the properties of the bodies of which it 
forms a part. For instance, if the molecules of gases had 
the form of long cylinders or rods all of the properties of 
the gases which depend upon the ease of movement of 
the molecules among themselves would be different from 
what they would be if the molecules were spherical. 
Indeed, it is impossible to calculate the characteristic 
1 'constants" of gases on the basis of the molecular theory 
without the use of some definite assumption about the 
shape of its molecules. The one usually made is that 
the molecules are spherical, and since it leads to general 
results which are in harmony with the facts it must be 
nearly correct. 

When a number of bodies are thrown together in a heap 
the size of the heap will depend not only upon the volume 
of the individual bodies and upon the number present, 
but also upon their shape. Thus, if they were all cubes 
and were packed neatly together they would fill the whole 
space marked off by the "heap." However, if they were 
spheres they could not possibly be arranged so as to take 
up all of this space. 

Now we have a number of means of estimating the 
volume of the individual atoms or molecules of substances, 
which are independent of any knowledge of the volume 
occupied by large masses of these substances. If we as- 
sume that the molecules are spherical, therefore, we can 
calculate the amount of space which should be filled by 
a body containing a given number of them under certain 



conditions, as for example those which hold at absolute 
zero, when the molecules are motionless. We can then 
compare this volume with that actually occupied under 
these conditions by a body made up of the same number 
of molecules of the special substances which we are 
studying. If the two are essentially the same we shall 
have some reason for supposing that our original assump- 
tion concerning the roundness of the molecules or atoms 
was correct. Calculations of this sort have been made 
and indicate that most atoms are either spherical or are 
slightly flattened spheres. 

The " Saturn fan Atom." Although it is desirable for 
purposes of clearness to think of the atom as possessing 
a definite surface having a definite contour, it is also im- 
portant to bear in mind the fact that the resemblance 
between an atom and a solid ball probably amounts to 
little more than an analogy. As represented in the fig- 
ures in the text, the surfaces or " edges " of an atom are 
probably ill defined, and they may even be changeful. 
Certain modern considerations which are touched upon 
in Section 53 make it seem altogether likely that the 
atom is really a system of rapidly rotating particles which 
are very much smaller than the atom itself. If this theory 
should prove to be true the size and shape of the atom 
would merely be those of the " orbits" of the outermost 
of these rotating particles, just as the size and shape of 
the solar system depends upon those of the orbits of the 
outermost planets. 

Some of the most recent studies on the shape of the 
atoms have been made by R. D. Kleeman. 


Kleeman's discussion of atomic shapes is rather difficult, but 
can be found in the Philosophical Magazine for July, 1910, vol. 20, 
pp. 229-238. 



[Sec. 5 

As an illustration of the manner in which the assumption of 
the sphericity of the atom or molecule enters into the theory of 
gases see: W. P. Boynton's "Applications of the Kinetic Theory," 
(1904) Chapter IE. 

A very popular discussion of " Solar system " conception of the 
atom is given in Chapter IV of C. R. Gibson's "Scientific Ideas 
of To-day," (1909) pp. 52-58. 

Section 5 


The official table of the chemical elements now con- 
tains eighty-three names, each of which corresponds with 
a particular species of atom. In addition to these we must 
include in our list of atomic species the many unstable 
elements, which have been discovered by the new science 
of radio-activity. 

The first table below gives the names of the recognized 
chemical elements with their " atomic weights" on the 
basis of oxygen = 16. The second table performs the 
same service with respect to the new radio-active ele- 
ments, several of which are also included in the first 


Atomic Name of 
Number Element 

1 Hydrogen . 

2 Helium 

3 Lithium . . . 
Glucinium . 


Carbon . . . 
Nitrogen. . . 
Fluorine . . . 






Sulphur . . . 
Chlorine. . 


.. H 


. .Gl 







. . Me 
. . .Al 

. . .P 









Nature under Ordinary Date 

Conditions Discovered 

Very light gas, chemically active. . 1766 

Very light gas, chemically inert . . . 

Light alkali-forming metal ........ 

Light metal, resembling zinc ..... 

Non-metal, occurs in "borax" ..... 

Non-metal (charcoal and diamond) 
Atmospheric gas, inert ........... 

Atmospheric gas, chemically active . 
Excedingly corrosive gas ......... 1810 

Inert atmospheric gas ............ 1898 

Alkali metal, occurs in "salt " ____ 1807 

White, combustible metal ........ 1808 

Light, white metal .............. 1828 

Hard, semi-metallic substance ____ 1823 




Inflammable non-metal 
Inflammable non-metal 
Yellow, corrosive gas 
Inert atmospheric gas 





Sec. 6] 


19 Potassium K 39.10 Alkali metal, occurs in "potash" . . . 1807 

20 Calcium Ca 40.07 Alkaline earth, occurs in "lime" . . 1808 

21 Scandium Sc 44.1 Metal found in rare earths 1879 

22 Titanium Ti 48.1 Metal found in rare earths 1796 

23 Vanadium V 51.0 Metal found in rare earths 1801 

24 Chromium . . . . Cr 52.0 Hard metal forming highly colored 

compounds 1797 

25 Manganese . . . Mn 54.93 Grayish metal 1774 

26 Iron Fe 55.84 Grayish metal P 

27 Cobalt Co 58.97 Silvery-white metal 1733 

28 Nickel Ni 58.68 Hard, silvery-white metal 1751 

29 Copper Cu 63.57 Reddish-yellow metal P 

30 Zinc Zn 65.37 Silvery-white metal 1520 

31 Gallium Ga 69.9 Hard, grayish-white metal 1875 

32 Germanium. . .Ge 72.5 Grayish, metallic solid 1886 

33 Arsenic As 74.96 Brittle, steel-gray semi-metallic subs. 1649 

34 Selenium Se 79.2 Sulphur-like solid 1817 

35 Bromine Br 79.92 Heavy, corrosive, red liquid 1826 

36 Krypton Kr 82.92 Inert atmospheric gas 1897 

37 Rubidium Rb 85.45 Soft, white, alkali-metal 1868 

38 Strontium Sr 87.63 Alkaline-earth metal 1808 

39 Yttrium Yt 89.0 Dark gray metal 1823 

40 Zirconium Zr 90.6 Hard, brittle, iron-gray metal 1824 

41 Columbium . . . Cb 93.5 Steel-gray metal 1846 

42 Molybdenum .Mo 96.0 Metal resembling iron 1782 

44 Ruthenium . . . . Ru 101.7 Platinum-like metal 1844 

45 Rhodium Rh 102.9 "Infusible," silvery metal 1804 

46 Palladium Pd 106.7 Fusible, platinum-like metal 1803 

47 Silver Ag 107.88 Metal, of familar properties P 

48 Cadmium Cd 112.40 White, malleable, zinc-like metal . . . 1817 

49 Indium In 114.8 Soft, malleable, zinc-like metal 1863 

50 Tin Sn 119.0 Silver white, very malleable metal . . P 

51 Antimony Sb 120.2 Brittle, white, crystalline metal 1450 

52 Tellurium Te 127.5 White, semi-metallic solid 1782 

53 Iodine I 126.92 Volatile, brown, non-metallic solid. . 1811 

54 Xenon Xe 130.2 Inert atmospheric gas 1898 

55 Caesium Cs 132.81 Soft alkali-metal 1860 

56 Barium Ba 137.37 Soft silver-white alkali-earth metal . 1808 

57 Lanthanum. . .La 139.0 Malleable metal found in rare earths. 1841 

58 Cerium Ce 140.25 Rare-earth metal resembling iron . . . 1801 

59 Praseodymium Pr 140.6 Rare earth metal 1885 

60 Neodymium. . .Nd 144.3 Rare earth metal 1885 

62 Samarium . . . . Sa 150.4 Rare earth metal 1879 

63 Europium Eu 152.0 Rare earth metal 1901 

64 Gadolinium. . .Gd 157.3 Rare earth metal 1886 

65 Terbium Tb 159.2 Rare earth metal 1843 

66 Dysprosium . . .Ds 162.5 Rare earth metal 1907 

67 Holmium Ho 163.5 Rare earth metal 1886 

68 Erbium Er 167.7 Rare earth metal 1843 

69 Thulium Tm 168.5 Rare earth metal 1879 

70 Ytterbium . . . . Yb 172.0 Rare earth metal 1878 

71 Lutecium Lu 174.0 Rare earth metal 1908 

73 Tantalum Ta 181.5 Hard, silvery, ductile metal 1802 

74 Tungsten W 184.0 Brittle, very " infusible, " metal 1783 

76 Osmium Os 190.9 Blue-gray resistant metal 1804 

77 Iridium Ir 193.1 Brittle, " infusible," metal 1804 

78 Platinum Pt 195.2 Silvery, " infusible," metal 1500 

79 Gold Au 197.2 Chemically resistant metal P 

80 Mercury Hg 200.6 Liquid, silvery metal 300 B. C. 

81 Thallium Tl 204.0 Soft, whitish metal 1861 

82 Lead Pb 207.10 Soft, bluish-white metal P 

83 Bismuth Bi 208.0 Very brittle reddish-white metal. . . . 1450 

86 Niton Nt 222.4 Inert gas (radium emanation) 1900 

88 Radium Ra 226.4 Radio-active alkaline-earth metal . . 1898 

90 Thorium Th 232.4 Heavy, rare-earth metal 1828 

92 Uranium Ur 238.5 Hard, white, malleable metal 1780 



The atomic weights above are those given by the 
"International Committee on * Atomic Weights,' for 
1914." "P" in the last column stands for "prehistoric." 


Atomic Name of Atomic Chemical 

Number Element Weight Analogue 

92 Uranium 1 238.15 Tungsten (h) 

90 Uranium Xi 234 Thorium (i) 

91 Uranium X2 234 Tantalum (h) 

92 Uranium 2 234 Uranium 1 (i) 

90 Ionium 230 Thorium (i) 

88 Radium 225.95 (HBnigschmid) .... Barium (h) 

86 Radium Emanation (Niton) . . .222 Xenon (h) 

84 Radium A 218 Polonium (i) 

82 Radium B 214 Lead (i) 

83 Radium C 214 Bismuth (i) 

84 Radium C' 214 Polonium (i) 

81 Radium C 2 210 Thallium (i) 

82 Radium D (Radio-Lead) 210 Lead (i) 

83 Radium E 210 Bismuth (i) 

84 Radium F (Polonium) 210 Tellurium (h) 

82 Radium G (probably Lead) . . 207.10 (?) 


90 Thorium 232.4 Cerium (h) 

88 Meso-Thorium 1 228 Radium (i) 

89 Meso-Thorium 2 228 Actinium (i) 

90 Radio-Thorium 228 Thorium (i) 

88 Thorium X 224 Radium (i) 

86 Thorium Emanation 220 Niton (i) 

84 Thorium A 216 Polonium (i) 

82 Thorium B 212 Lead (i) 

83 Thorium C 212 Bismuth (i) 

84 Thorium C' 212 Polonium (i) 

81 Thorium D 208 Thallium (i) 


89 Actinium 230 (?) Lanthanum (h) 

90 Radio-Actinium 230 (?) Thorium (i) 

88 Actinium X 226 (?) Radium (i) 

86 Actinium Emanation 222 (?) Niton (i) 

84 Actinium A 218 (?) Polonium (i) 

82 Actinium B 214 (?) Lead (i) 

83 Actinium C 214 (?) Bismuth (i) 

81 Actinium D 210 (?) Thallium (i) 

Only ten of the thirty-five substances named above 
are believed to represent chemically new atomic species. 
The atomic weights of only a few of them are accurately 



known ; those of the others are estimated on the basis of 
the theory of radio-active disintegration. The last column 
gives the name of another element to which the given 
element is closely similar. In the cases marked (i) the 
two elements are "isotopic" i.e., although of different 
atomic weight or radio-activity, are identical in chemical 
character, (h) indicates a "homologue" in the periodic 

The weights of the atoms are expressed in terms of 
the weight of the lightest atom, namely that of hydro- 
gen. 1 Thus, when we say that the "atomic weight" of 
oxygen is 16 we mean that an atom of oxygen weighs 
16 times as much as an atom of hydrogen. 

Method of Finding Atomic Weights. The methods by 
which the relative weights of the atoms are determined 
are not difficult to understand. Suppose, for example, 
that the chemist desires to learn the atomic weight of the 
element chlorine. He knows by experiment that chlorine 
gas will combine with hydrogen gas in definite propor- 
tions to form hydrochloric acid. When he analyzes this 
compound he finds that it contains 35.18 parts of chlorine 
by weight to one part of hydrogen. Consequently on the 
assumption that the molecule of hydrochloric acid is made 
up of one atom of hydrogen and one atom of chlorine, he 
is able to determine the atomic weight of chlorine as 35.18, 
assuming that of hydrogen to be unity (or as 35.46 on 
the basis of the weight of oxygen taken equal to 16). 
Similar measurements can be made of the proportions by 
weight in which other elements combine with hydrogen, 
and these will furnish a basis for the calculation of their 
atomic weights also. 

1 As a matter of fact, the unit atomic weight now generally used 
is T V the weight of the oxygen atom, but this differs only slightly 
from that of the atom of hydrogen. 



It may well be asked how it is that the chemist can 
know the number of atoms of particular elements which 
are contained in a given compound unless he first knows 
the atomic weights of these elements. For example, the 
compound, water, is made up of one part of hydrogen to 
eight of oxygen, but instead of stating on the basis of this 
fact that the atomic weight of oxygen is 8 the chemist 
comes to the conclusion that water contains two atoms of 
hydrogen, so that the atomic weight of oxygen is 16. 
One of the things which leads him to this conclusion is 
the fact that if he takes the atomic weight of oxygen as 8, 
he will soon find it necessary to suppose that the mole- 
cules of certain substances other than water, contain 
oxygen in less than atomic quantities, which is impossible. 
Thus by comparing many different results derived from 
the analysis of different compounds containing the same 
elements, the chemist is led finally to the selection of 
relative weights in harmony with the atomic theory. 

However, there are other considerations which lead to 
similar results. It is a well-known principle that equal 
volumes of all gases under the same conditions contain 
the same number of molecules, and from this fact it fol- 
lows that if we weigh equal volumes of different gases 
under these constant conditions we will obtain, imme- 
diately, the relative weights of their constituent mole- 
cules. Thus, when water is broken up into hydrogen and 
oxygen gas it yields two volumes of the former for one of 
the latter, a fact which seems to indicate that twice as 
many atoms of hydrogen are present in a molecule of 
water as there are atoms of oxygen. It is obvious that the 
" molecular weights" of all substances which are capable 
of being converted into a gas can be found by measuring 
the weight of a unit volume of this gas under standard 
conditions. Such a knowledge of the relative weights of 



the molecules composing a compound will clearly be 
of the utmost value in the determination of the weights 
of the atoms of which they in turn are composed. 

Some of the elements do not combine with hydrogen to 
form substances which it is convenient to analyze. How- 
ever, the atomic weights of such elements may be found 
indirectly by studying the ratios in which they unite with 
other elements of known atomic weight. Thus, most 
elements which have slight affinity for hydrogen combine 
very readily with oxygen, and since hydrogen also forms 
a definite compound with oxygen, this latter element 
constitutes a connecting link between hydrogen and the 
element whose atomic weight we desire to determine. 

Atomic Weights and Atomic Volumes. The idea has 
recently been advocated by J. Traube that a simple and 
definite relationship exists between the size of an atom 
of any element and its weight. The volume of the atom 
appears to vary approximately as the square-root of its 
weight. This means that the diameter of a spherical atom 
would be proportional to the sixth root of its weight, so 
that although atomic diameters vary rather widely among 
the lighter elements the diameters of the heavier atoms 
are very nearly alike. This law connecting atomic weights 
and volumes is based upon measurements of essentially 
the same character as those which we have discussed in 
Section 2, above. However, it appears also to be in har- 
mony with certain phenomena in radio-activity which 
seem to depend upon the volume of the atoms composing 
a body. 


A table of the radioactive substances, with their properties, will 
be found in Kaye and Laby's " Tables of Physical and Chemical 
Constants" (1911), pp. 107-108. A more elaborate discussion will 
be found in Ernest Rutherford's "Radio-Active Substances and 
Their Radiations," (1913). For the most recent developments see 



the 1915 edition of Frederick Soddy's "The Chemistry of the 
Radio-Elements," pp. 70-142. 

On the choice of atomic weights, see Wilhelm Ostwald's " Out- 
lines of General Chemistry" (1890), Part I, Book VI, Chap. I, 
pp. 178-182. 

An excellent detailed discussion of the determination of atomic 
weights also appears in H. C. Jones' "The Elements of Physical 
Chemistry" (1902), pp. 4-18. 

Section 6 

Systematic Relations between Elements. We might 
conceive a state of affairs in which all of the chemical ele- 
ments would be wholly different from one another, but 
everyone knows that this is not actually the case in na- 
ture. When the elements are compared it is found that 
curious resemblances exist among them, and moreover 
that these resemblances depend in some way upon the 
atomic weights of the elements which are compared. 
Take for example the three elements, chlorine, bromine 
and iodine, which everyone who has handled them in 
the chemical laboratory knows to be curiously similar. 
We find upon investigation that the atomic weight of 
bromine is approximately the arithmetic mean between 
those of chlorine and iodine. Several other almost equally 
striking " triads" (or groups of three) of this character 
can be pointed out. 

The systematic distribution of the properties of the ele- 
ments becomes still more obvious when we arrange them 
in order of their atomic weights, as hi the table below. 
It then appears that, with certain exceptions, every 
eighth element in the series possesses similar properties. 
This fact is indicated in the table by placing the names 
of the elements which resemble each other in the same 



vertical columns. Such elements are said to belong to 
the same "family," or " group," and it will be observed 
that there are eight families of this sort in the table. All 
members of one family tend to have the same combining 
power, or "valency" (see Section 34, below), and their 
compounds with other elements tend to resemble each 
other closely. Thus hi the second column, lithium, so- 
dium, potassium, rubidium and caesium are all metals 
forming strongly alkaline compounds with hydrogen and 
oxygen, of which ordinary "washing soda" is an ex- 
ample. Each of these elements has normally a combin- 
ing power of one, that is one atom of each of them will 
unite with one atom of such an element as chlorine, which 
in turn unites with one atom of hydrogen. As we pass 
from left to right in the table the combining power of the 
represented elements increases and then diminishes, 
periodically. Copper, silver and gold which are classed 
in the second family differ from the other five elements 
in the family but form a transition group to those in the 
third column. 

Nearly all of the properties of the elements are found 
to depend upon their position in the "periodic table," 
as the arrangement which we are discussing is called. 
This is true not only of the chemical properties but also 
of such physical properties as melting point, specific 
gravity, etc. 

The elements of a single horizontal line are said to be 
members of the same "series" or "period." The table 
is called "periodic," for the reason that as we pass from 
one member of a "series" to the next, the properties of 
the elements first depart from those of the element with 
which the series started and then alter their course of 
variation to return to properties closely resembling those 
of the first member of the series. 



Defects in the Table. It will be noted that certain 
blank spaces occur in the table as we have represented it. 
These spaces are left vacant for occupancy by elements as yet 
undiscovered. When the table was first constructed by 
Mendelejeff there were more empty spaces in it than 
there are now. The faith of chemists in the table was so 
great that they were led to predict the discovery of ele- 
ments having atomic weights permitting them to fit into 
these vacant spaces, and they even went so far as to 
specify the properties of these elements and of then* 
compounds. In several cases these predictions have 
since been fulfilled by the actual discovery of the ele- 
ments in question. 

As can be seen by inspection the periodic table is not a 
perfect system for the classification of the elements, or 
if so is far from simple in many respects. All of the ele- 
ments do not fit nicely into their places, this being es- 
pecially true in the case of the nine which are represented 
in the ninth column. Here we have groups of closely sim- 
ilar elements which are of about the same atomic weight 
and which refuse to fit into the scheme of " families." 

Significance of the Periodic Relationships. The exact 
meaning of the definite relationships between the ele- 
ments which are proven by the periodic table to exist, 
is at present largely a mystery. However, there can be 
little doubt that the demonstrated resemblances depend 
in some way upon the constitution of the atoms of which 
the elements are made up. (See Section 53, below.) 
The fact that the properties of the elements appear to be 
determined (at least approximately) by the relative 
weights of their atoms clearly suggests this interpreta- 
tion, for if the atoms have a definite internal structure and 
if the units of this structure are similar for all of the atoms, 
increasing complexity would necessarily mean increasing 



atomic weight. If the units of structure were not similar 
we should hardly expect to find the systematic resem- 
blances which actually exist. 

One of the earliest theories to make use of the idea of 
a single substance or "protyle" common to all of the 
chemical elements was that of Prout, who suggested in 
1815, that the heavier atoms might be clusters or con- 
densations of various numbers of hydrogen atoms. This 
hypothesis received support from the fact that a large 
number of atomic weights more than would be ex- 
pected as a result of chance are approximately integral 
multiples of the weight of the hydrogen atom. However, 
accurate determinations of the weights which fail to 
approximate such values did not substantiate Prout's 
original belief that the deviations were due to experi- 
mental errors. Consequently, in its primitive form, at 
least, the theory had to be rejected. 

The new facts brought to light in the study of radio- 
activity (see Section 46, below), however, make it prac- 
tically certain that if hydrogen is not one of the funda- 
mental bricks of which the heavier atoms are built, just 
such a role is played by the element helium. In radio- 
activity, atoms break down into others of less weight, and 
this loss of weight always occurs in single decrements of 
four units. This is the mass of the helium atom, and it 
has been proved that every change in mass of a radio- 
active substance is accompanied by the generation of 
helium. Besides this, it has been pointed out that a 
difference of four units between neighboring members in 
the periodic table is a very common one. 

The Nucleus Theory of the Atom, and Isotopes. How- 
ever, this last-mentioned rule is far from being strictly 
obeyed, and it is certain that hydrogen itself cannot be 
made up of helium. At the present time there is current 



[Sec. 6 



p* 1 


^ 1 




l 5 

w > 
















w w 










10 "P. ^ 



eo g 10 
co w c- 







| a 


8 f2 























w w 










w w 







- 1 










4J 00 



s ^ 

M o 












Sec. 6] 


3 10 




w *" 






























m ^ 

S 2 











ft g. 


K 9 






a definite and well-grounded conception of the structure 
of the hydrogen atom (Cf. Section 53), in accordance 
with which the latter is composed of a single minute 
particle of negative electricity an electron and a very 
much smaller particle of positive electricity about which 
the electron revolves. Practically the entire mass of the 
atom is concentrated in the positive "nucleus," as it is 
called. The more ponderous atoms are supposed to be 
formed from a larger number of positive and negative 
particles, according to their respective weights. The 
positive particles are always condensed into very small 
nuclei, together with a portion of the electrons, and it is 
probable that, hi the case of the heavier elements at least, 
the helium atom is an important secondary unit in the 
structure of these nuclei. 

Within the past few years the study of radio-active sub- 
stances has brought out facts which indicate that the 
fundamental principle of the periodic table that the 
chemical properties of an element are determined by its 
atomic weight is only approximately true. As already 
mentioned (Section 5, above), radio-active elements 
isotopes are known which differ in atomic weight but 
have identical chemical properties. On the other hand, 
there are elements of the same atomic weight which show 
radical differences chemically. The explanation of this 
very important discovery apparently lies in the sugges- 
tion that the chemical nature of an atom depends not 
directly upon its weight but upon its electrical structure, 
which to a limited extent may change practically inde- 
pendently of the weight. Of this more will be said in the 
appropriate context. (See Section 53.) 

Chemical Elements as Atomic Mixtures. The exist- 
ence of " isotopes" among the radio-elements suggests 
their presence in other parts of the periodic table. In 



the latter case there appears a possible solution of the 
difficulty that the majority of the atomic weights are not 
accurate multiples of that of hydrogen. It is conceivable, 
even probable, that the chemical elements which we 
have previously regarded as individual species of matter 
are in reality only types, or classes, of such individuals, 
all of the members of a single class being indistinguish- 
able and non-separable from each other by purely chemi- 
cal means. In this case the atomic weights given in 
Table I (Section 5) must represent averages of the rela- 
tive weights of a number of chemically similar atoms 
with different masses, which may be present in standard 
chemical preparations in nearly constant but in unequal 
amounts. If this is true it is not surprising that our em- 
pirically determined atomic weights do not have integral 

The above speculations receive support from certain 
experiments by J. J. Thomson, not depending in any way 
upon radio-active phenomena, which suggested that the 
inert atmospheric gas, neon, is really made up of two 
isotopic constituents, one of atomic weight about 20 and 
the other (meta-neon) about 22. F. W. Aston, following 
out this clue, found that by the use of physical methods 
relying on the relative rates of diffusion of the two 
components, pure atmospheric neon could actually be 
separated into two gases of different density. Careful 
comparisons of samples of lead derived from geologically 
different sources indicate that this element, also, may 
be a mixture of isotopes, an idea which is strongly sug- 
gested by its relation with the radio-active substances. 

All of these results should be accepted with reserva- 
tions on account of their novelty, but it must be admitted 
that they open a vista of new insights into the meaning of 
the periodic table. 




For a detailed discussion of the periodic table see Harry C. 
Jones, "The Elements of Physical Chemistry" (1902), pp. 18-37. 

A more popular exposition can be found in R. K. Duncan's "The 
New Knowledge " (1905), Part H. 

See also W. Nernst's: "Theoretical Chemistry," Eng. trans. 
from 6th German ed. (1911), pp. 178-190. 

On "isotopes," see Frederick Soddy's "The Chemistry of the 
Radio-Elements" (1915), Part I, pp. 50-56; Part II, complete. 
Also an article by E. Rutherford: "The Constitution of Matter," 
Popular Science Monthly, August, 1915. 

Section 7 


Atoms may group themselves to form molecules of 
almost any conceivable shape. The properties of the 
substance which such molecules compose seem to depend 
in large part upon the manner in which the atoms are 
combined) that is, not only upon the number and nature 
of the atoms but upon their geometrical arrangement 
within the molecule. 

To show what a great variety of substances can be 
formed by different modes of combination of the same 
elements it may be stated that about two hundred thou- 
sand distinct compounds of the element carbon are now 
known, most of which are with only three other elements : 
hydrogen, oxygen, and nitrogen. These substances be- 
long to the class of "organic" compounds, so called for 
the reason that many of them are essential in the 
chemical structures and changes of living organisms. 
There are undoubtedly thousands, if not millions, of 
specific chemical substances in living bodies which are 



built up from the same four elements but which have 
not yet been separated out and analyzed. 

The element carbon is remarkable on account of the 
very large number of compounds which it can form, but 
the other elements also enter into the composition of a 
great variety of different substances. Each of these 
substances, organic or inorganic, possesses characteris- 
tic properties which distinguish it more or less sharply 
from all other substances. 

homers and Structural Formulae. That the differ- 
ences which exist in the properties of substances must 
depend at least in part upon the arrangement of the atoms 
within the molecule is proved by the fact that compounds 
exist which have quite different properties but exactly 
similar numbers and kinds of constituent atoms. Such 
cases are quite common in organic chemistry, and the 
substances involved are known as "isomers." Thus 
the organic chemist is acquainted with twenty-six different 
compounds which contain four atoms of carbon, six of 
hydrogen and three of oxygen, and with one hundred and 
fifty-seven which are composed of ten atoms of carbon 
and sixteen of hydrogen. 

It is customary for the chemist to represent the make- 
up of a compound by means of a so-called "chemical for- 
mula" which shows in a simple way the constitution of the 
molecules of the substance. The formula of water, H 2 O, 
may be taken as a simple example. This formula states 
that a molecule of water is made up of two atoms of hydro- 
gen, H, combined with one atom of oxygen, O. As an 
example of a more complex formula we may consider 
that of alcohol, C 2 H 6 O, or that of the coal-tar oil, benzene, 
C 6 H 6 , in both of which C stands for the element, or the 
atoms of carbon. 

There is only one substance which has the formula 


H 2 O, so that no confusion can arise as to the meaning 
of this formula. It always stands for water. However, 
at least two substances are known which have the 
formula C2H 6 O, namely ordinary alcohol, and a gas called 
* 'methyl ether," which is closely similar to the ether 
employed in surgical operations. It is obvious that if we 
desired we could write the formula of water as H-O-H, 
in order to show the probable manner in which the atoms 
are combined in the molecule, but although this is not 
required in the case of water it is found necessary hi the 
case of alcohol, and other substances which have isomers. 
Such a formula is called "structural" or "graphical" 
because it is a simple drawing representing the supposed 
arrangement of the atoms in the molecule of a substance. 
These formulae can be constructed by studying the chemi- 
cal relationships which exist between different com- 
pounds, and when thus evolved they not only enable us 
to distinguish theoretically between isomers, but also 
explain the differences which are found hi their properties. 
However, as we shall see, their utility is not limited to 
the study of isomerism, for it is obvious that the more 
exact is our knowledge of the structure of different mole- 
cules the clearer will be our ideas concerning the changes 
which are liable to occur in these molecules. 

A study of the manner hi which alcohol can be built 
up from its elements leads us to assign to it the structural 


I I 

formula: H-C-C-O-H. The substance "methyl ether," 
I i 

on the other hand, which we have mentioned as an isomer 

H H 

I | 

of alcohol, proves to have the formula: H-C-O-C-H. 

[ 78 ] H H 


H H H H H H 
Normal Hexane H C C C C C C H 

H H 

H C C H 

H | 

I H 

Methyldiethyl Methane H C C H 

H I 

H C C H 

I I 

H H 


H C H H H H 

Dimethylpropyl Methane H C C C C H 

H C H H H H 


H H 

H C H H C H 

I | 

Dimethylisopropyl Methane H C- C H 

H C H H C H 


H C H 
H H 

Trimethylethyl Methane H C C - C H 


H C H 

H C H 


Fig. 12 


These chemical formulas represent the supposed structure of the mole- 
cules of five distinct substances all of which contain the same number of 
hydrogen and of carbon atoms. 



It is easy to see why the decomposition of the two mole- 
cules thus represented should lead to different results in 
spite of the fact that identically the same atoms are 
present in each case. We believe that these formulae 
give a partial representation of the actual arrangement 
of the atoms in the molecules of alcohol and methyl ether 

Figure 12 gives the graphical formulae of five isomeric 
hydrocarbons each made up of six atoms of carbon, 
and fourteen of hydrogen. Although the formulae show 
certain general resemblances, the five structures are 
nevertheless quite distinct. The resemblances corre- 
spond with an actual physical similarity of the sub- 
stances, which causes them to be grouped in the same 
general class, but within this class the substances show 
a perfectly clear chemical individuality. Many other 
examples of this principle that the nature of a chemical 
substance depends upon the exact geometric structure 
of its molecules could easily be found. 

The "Benzene Ring" - The approximate truth of the 
representations of the structure of molecules which are 
given by the structural formulae now in use among chem- 
ists, is attested by such cases as that of the formula of 
benzene, the coal-tar oil of which we have spoken above. 
The six carbon atoms are supposed to be arranged in a 
molecule of this substance in the form of a ring, and to 
each of them is attached one of the hydrogen atoms. 
This formula is shown in Figure 13, (a). Each of the 
hydrogen atoms in the molecule can be replaced by atoms 
of other elements, as for example chlorine atoms, and for 
every new and different molecule thus produced there 
should exist a correspondingly distinct substance, which 




H C C H H C C Cl 

H-C C-H H-C C-H 
V V 

C. H 

C t 

X \ / \ / \ 

Cl C C H Cl C C Cl Cl C C H 

H C C H H C C H H C C Cl 

V V V 

i Jl 


t t t 

X \ / \ / \ 

Cl C C Cl Cl C C Cl Cl C C Cl 

H C C Cl H C C H H C C H 

V V V 


Fig. 13 a 
See Fig. 13 b 





/ \ / V / \ 

H C C Cl H C C H H C C Cl 

H C C Cl Cl C C Cl Cl C C H 

V V 





/ \ / \ 

H C C Cl Cl C C Cl 


Cl C C Cl Cl C C Cl 

\ X V X 

C C 


Fig. 13 b 

The significance of these formulae is explained in the text. 

would be known to the chemist as a "chlorine derivative 
of benzene." 

Now a moment's study will show that by a simple exami- 
nation of the ring formula we can predict the number of 
such derivatives which we shall be able to form, provided 
the formula is correct. If only one hydrogen atom is 
replaced there is only one possible compound, (b) Figure 
13, since the ring is perfectly symmetrical and hence the 
structure which is formed is the same no matter which 
hydrogen atom is disturbed. However, if two, three, or 



four hydrogen atoms are replaced there will be three dif- 
ferent molecular structures corresponding to each of these 
numbers. This fact is shown in Figure 13 (c) to (k) 
inclusive. No more than three can be formed, however, 
in each case. If five or six chlorine atoms are introduced 
only one compound can be produced corresponding to 
each number, (1) and (m), respectively, in the Figure. 
We are thus able to prophesy the possibility of twelve 
chlorine derivatives of benzene and of no more than 
twelve. The actual study of this substance in the labo- 
ratory has revealed the existence of all of these deriva- 
tives and has proven the impossibility of producing any 
others. This is a striking verification of the idea that the 
benzene molecule actually has a ring structure. 

We have studied this matter of the structure of the 
molecule in connection with benzene because the formula 
of this substance is distinctive and is one of the most 
successful in its applications. However, there are other 
instances of the same thing which are almost equally 
striking. Indeed the science of organic chemistry would 
be practically impossible without the help which is pro- 
vided by a knowledge of the actual structure of the mole- 
cules composing the substances with which it deals. The 
exact structure of the molecule is of less importance hi 
inorganic chemistry because here the molecules are so 
much simpler. 

Molecules of Single Elements. In this connection it 
may be well to note the fact that the atoms of the elements 
in the pure state generally unite to form molecules, which 
are thus made up of two or more atoms of the same kind. 
Thus hydrogen gas is not composed of free atoms but of 
hydrogen molecules, each of which contains two atoms of 
the element. Many simple elements in gaseous form 
have two atoms hi then* molecules. The vapor of the 



[Sec. 7 

metal mercury is distinguished from most elementary 
gases by the fact that its atoms are uncombined. Some 
elements, on the other hand, form molecules containing as 
many as seven similar atoms, and the same element may 
yield molecules of different sizes under different condi- 


Fig. 14 


To gain a correct impression from this drawing one should imagine the 
H and OH circles on the inner portion of each model to be spheres pro- 
jecting outward from the page, so that the models have a three-dimen- 
sional form. The letters attached to each black circle stand for the groups 
of atoms which the circle represents, and the lines connecting the circles 
indicate the structure of the molecules. It will be observed that these 
two molecules, which are of "right "and " lef t " tartaric acid respectively, 
are so constructed that one is the mirror-image of the other. This rela- 
tionship of structure is offered as an explanation of the similar relationship 
which exists between the structure of the crystals shown in Figure 15. 

tions. The various forms of pure sulphur and of phos- 
phorus the yellow and the red probably correspond 
to molecules containing different numbers of atoms of 
these elements. It is possible that the three familiar 


Sec. 7] 


forms of the element carbon: charcoal, graphite, and 
diamond, may be due to the same causes. 

Molecular and Crystal Structure. The structure of the 
molecule which is characteristic of a substance is proba- 
bly closely related with the shape of the crystals which 
it produces. 1 Nearly all pure substances will take on a 
characteristic crystalline shape under the right condi- 
tions. There are certain pairs of sugars compounds of 


Fig. 15 


It will be observed that the two crystals represented above are identical 
inform except for the fact that one is the mirror-image of the other; what 
is on the right-hand side of one is on the left-hand side of the other. The 
two crystalline forms represent two different kinds of tartaric acid, but 
ordinary chemical analysis reveals no difference in their composition. 
It is supposed that the actual basis of the distinction lies in the fact that 
the molecules of the two acids differ in the same general way in which 
the crystals differ. These molecules are symbolized in Figure 14. 

carbon, hydrogen, and oxygen the molecules of which 
as represented in their structural formulae, are distin- 
guished from each other only by the fact that one is the 
mirror-image of the other. This difference is made clear 
in the accompanying Figure 14. Now it turns out that 
when these sugars crystallize, although the crystals do 
not have the same form as the molecules, they do differ 

1 Present-day studies of crystal constitution (see Section 22) 
show that the atom and not primarily the molecule is the unit of 



in the same way in which the molecules differ, i.e., one is 
the mirror-image of the other. This fact, which is shown 
by a comparison of Figures 14 and 15, seems to prove 
quite conclusively that the shape of the crystal depends 
directly upon that of the molecule. 

In this connection it is interesting to note that whereas 
one of these sugars either in the crystalline form or in 
solution turns polarized light to the left, the other 
turns it to the right, and in exactly the same proportion. 
This fact speaks for the truth of the formulae which have 
been assigned to the compounds. 

Further considerations with regard to crystal structure 
will be found in Section 22. 


An excellent, detailed and not very difficult discussion of "The 
Constitution of the Molecule," will be found in W. Nernst's 
" Theoretical Chemistry," English translation from 6th German 
edition (1911), Book H, Chapter 4, pp. 278-300. 

Simpler considerations appear in F. J. Moore's " Outline of 
Organic Chemistry" (1910). Chapter VIII, pp. 147-161, deals 
with the phenomena of crystal form above mentioned. 

Section 8 


Importance of the Internal Molecular Forces. We 
have asserted in Section 7, above, that the properties of 
compound substances depend principally upon the man- 
ner in which the atoms are arranged in the molecule. 
Strictly speaking, however, we must say that the char- 
acteristic properties of a substance depend upon the 
strength and arrangement of the forces of attraction which 
hold the molecule together. We perceive such qualities 
of bodies as hardness, elasticity, color, odor, etc., only 


Sec. 8] COLOR 

because these bodies bring to bear upon our organs of 
touch, sight, and smell certain characteristic combina- 
tions of forces. The nature of these forces must always 
depend at least in part upon the nature of the forces within 
and between the molecules of which the body is made up. 

Hard bodies are those in which the forces which exist 
between the molecules are very strong and hold them 
closely together, so that the body cannot be distorted by 
our touch. An elastic body is one in which the same 
forces act to restore its original shape, once it has been 
distorted. Among other characteristic properties of com- 
pound substances which must be determined by the 
internal and external forces of the molecule may be 
mentioned their melting and freezing points, their latent 
heats of vaporization and of fusion, their dielectric con- 
stants (or the degree to which they alter the intensity of 
an electrical field in which they are placed), their optical 
nature, their surface tension and the pressure exerted by 
their vapor when in the liquid state, their chemical activ- 
ity, the forms of then* crystals, then* compressibility, 
viscosity, magnetic power, etc. In the course of the dis- 
cussion in both Part I and II, the manner in which these 
properties are determined by molecular and inter-molec- 
ular forces will gradually be made clear. 

Color. The colors of substances depend upon the 
nature of the light which they absorb and reflect. If a 
body looks red in white light this means that it absorbs 
a great deal of green and blue light and reflects a rela- 
tively large amount of red. This power to absorb one 
light and to reflect another is known to depend directly 
upon the strength of the forces which hold the electrons 
in the molecule, as is explained in Section 41, below. 

The nature of the elements of which compounds are 
made up is undoubtedly of the utmost importance in 



determining their properties, but it is probable that the 
elementary atoms are effective primarily through their 
power to regulate the forces within the compound mole- 
cules. Thus the blue color of many copper compounds is 
due to the copper atom common to all. But since this 
color changes to red or brown when certain well-known 
changes occur in the forces binding the molecules of such 
compounds together, we are compelled again to conclude 
that what may be called the " dynamical (or force) con- 
stitution of the molecule" is the immediate cause of the 
physical properties of the corresponding substance. 

Allotropism. The marked differences which exist 
between the so-called "allotropic" forms of certain 
elements (such as carbon; see Section 7) obviously can- 
not be attributed to differences in the elementary con- 
stitution of the substances, and hence must be explained 
in terms of the different arrangement, and degree of ex- 
haustion, of the same atomic forces. Diamond which 
is one of the hardest substances known is made up of 
exactly the same element as charcoal and graphite, which 
are relatively soft. 

The Mystery of Chemical Change. Accordingly, re- 
flection should free us of the mystery which usually 
attaches to the qualitative modifications of the properties 
of bodies which occur in chemical changes. Molecules 
are not merely chance "heaps" of different atoms. 
They are definite and relatively stable individuals, the 
natures of which depend, of course, upon the forces latent 
in the atoms which make them up, but which neverthe- 
less realize in their own constitution " force patterns" 
which do not exist elsewhere. Each new atomic com- 
pound means a new system of such forces, and conse- 
quently a new substance, possessing an individuality of 
its own. 



Recent developments connected with the study of iso- 
topism (see Section 6), make it very probable that the 
principal physical and chemical properties of elementary 
substances depend directly only upon the number and 
arrangement of the electrons in the outer shell of an 
atom. This superficial structure is identical in isotopes, 
although the inner, nuclear formations differ. Now, there 
is little doubt that it is the outer or " valency" electrons 
(Cf. Section 34), which are active, and change their posi- 
tions, in chemical reactions. Consequently, it is natural, 
if what has just been said is true, that the properties of 
compounds should differ radically from those of the ele- 
ments which go to make them up. Only a relatively 
small portion of a complex atom is involved in its every- 
day dealings with the external world. The immediately 
interesting things about an atom, so to say, are all super- 
ficial, and are easily modified through intercourse with 
other atoms. From the point of view of chemistry, it is 
possible that the atom may be more radically altered in 
a chemical reaction than in a radio-active transformation, 
although the latter is fundamental and irrevocable, and 
the former easily reversible. 


The details involved in the above discussion will be further con- 
sidered in subsequent sections in which references will be given 
to the special topics concerned. 

For a more detailed general discussion see W. Nernst's "Theo- 
retical Chemistry" (1911), Book H, Chapter 5, pp. 303-347,. and 
Norman Campbell's "Modern Electrical Theory," second edition 
(1913), Chapter XII. 

A good discussion of color appears in Franklin and MacNutt's 
"Light and Sound" (1909), Chapter X. See also M. Luckiesh's 
excellent volume, "Color and its Applications" (1915). 



Section 9 

The chemist is accustomed to represent chemical 
changes by means of equations, such as the following : 

H 2 = 2 H + O, 

which is intended to show how water breaks down into 
hydrogen and oxygen. The symbols on the left-hand side 
of the equation represent the substances entering into 
the reaction, and those on the right-hand side represent 
its products. The equation above, stands for a chemical 
change of a purely destructive type. If the direction of 
the change is reversed, we have: 

2 H + O = H 2 O 

which is a constructive reaction. However, very few chem- 
ical changes are purely destructive or purely constructive. 
As a rule, there is simultaneous building up and break- 
ing down. Thus the actual process which occurs when 
water is decomposed into hydrogen and oxygen is not 
so simple as we have represented it in the first equation 
above, but is more accurately symbolized by the follow- 
ing relationship: 

2 H 2 O = 2H 2 + O 2 

This reaction probably goes on in two stages, the first 
being that indicated in the equation which was originally 
given, and the second being the combination of the free 
hydrogen and oxygen atoms thus produced, to form the 
hydrogen and oxygen molecules, H 2 and O 2 , respectively, 
which we mentioned in Section 7. However, since the 
chemist is usually interested in the so-called "end prod- 
ucts" of a reaction, and since in any case the two lines 



of change are continuous with each other, the reaction is 
ordinarily written as shown above. 

Section 10 

Everybody is aware of the fact that particles of matter 
attract each other with a force which is greater the nearer 
the particles are together. As everyone knows, it is the 
gravitational attraction between the earth and the bodies 
upon it which causes the latter to have "weight." Now 
since all bodies are made up of atoms it follows that the 
forces of gravity must depend in some way upon attrac- 
tions which atoms exert u'pon each other, and on account 
of the fact that the atoms are separated, at least in the 
case of solids and liquids, by very minute distances we 
should expect these " inter-atomic" forces to be relatively 
more powerful than are those of ordinary gravitation. 
But as far as the atoms are concerned gravitation is only 
a sample of much more powerful forces, for the former is 
in all probability a mere residue from the latter. 

At the present time the nature of the relationship which 
almost certainly exists between gravitation and the 
mutual attractions of the atoms is largely a mystery, but 
strange as it may seem, we are much clearer concerning 
the connection between atomic and molecular attractions, 
between "chemical affinity" and the forces of cohesion 
within bodies. 

As we have suggested in Section 6, the atoms are 
probably complex, being made up of ultimate particles 
which are much smaller than the atoms themselves. On 
account of the great stability of atoms we must suppose 
these particles to be held in position by very powerful 
forces of attraction. These forces, which are probably 



electrical, are not perfectly balanced within the atoms and 
hence tend to be effective in causing them to adhere to 
each other. Such secondary attraction is probably the 
basis of what is commonly called "chemical affinity," 
the force which binds atoms together into molecules. 
But just as the tendencies of attraction are not wholly 
exhausted within the atom, so there are residual attrac- 
tions which remain after the molecule has been formed, 
and it is these secondary residual forces which underlie 
the properties of cohesion, elasticity, and rigidity in 
solids or liquids. Their existence also accounts for cer- 
tain striking characteristics of gases, as well as of solids 
and liquids, as we shall see later on in our discussion. 

It is important that the reader should bear in mind the 
qualitative identity of these different forces of attrac- 
tion which operate between the particles of which all 
bodies are composed, and also the nature of their quanti- 
tative relationships. 


On the relation between atomic, chemical and cohesion forces, 
see Sir Oliver Lodge's book on "Electrons" (1906), Chapter XVI. 

Also Norman Campbell's " Modern Electrical Theory," second 
edition (1913), Chapters XII and XIII. 

Section 11 

The Nature and Foundations of the Theory. The 
" proof" of the doctrine that the atoms of all bodies are 
in constant motion has been given by the so-called " ki- 
netic molecular theory," hi connection with the results 
of experiment. This theory may well be described as an 
application of the laws of mechanics, or " dynamics," 
to the world of molecules. The fundamental principles 



of mechanics are the familiar "laws of motion" of New- 
ton, and it is a very significant fact that these principles 
which were first applied successfully to astronomical 
bodies should apply also to the almost infinitely smaller 
bodies called molecules. Certain very recent considera- 
tions tend to limit the applicability of these laws (see 
Section 54, below), but they certainly apply approxi- 
mately, and on the average, to a wide variety of molecular 

The kinetic theory regards each molecule as an inde- 
pendent being, endowed with motion and having certain 
attractions for all other molecules. It then proceeds to 
investigate the effects which should follow from the 
presence of a very large number of such molecules in a 
limited space, making use only of the laws of motion, of 
geometry, and of arithmetic. 

Molecular Chances, and Averages. It was shown by 
Maxwell, who may be considered the founder of the 
kinetic theory, that most of the laws which govern such 
a melee of vibrating particles must be "statistical" in 
character, that is, that they must depend upon the 
average of a large number of different individual molecular 
effects. Because of the vast number of molecules which 
are contained in even a small volume of material sub- 
stance, these averages are very certain. When a small 
number of molecules are considered, however, their 
action proves to be less certain. 

The modern theory of matter has thus given rise to 
a branch of inquiry which is often called "statistical 
mechanics," because it applies the principles of statistical 
investigation to mechanical problems. These principles 
are essentially those of "chance" or "probability." 
We can tell what will happen in the molecular world in 
about the same way in which we can predict the events 



which occur in human society, although generally with 
greater accuracy. It is possible for insurance men to 
calculate with sufficient accuracy for successful business 
administration, the number of people who will commit 
suicide or arson during a given period, or who will be 
killed in train wrecks or in automobile accidents. With 
regard to one person nothing definite could in general be 
foretold, but the greater the number of individuals con- 
sidered, the more precisely are predictions fulfilled. It 
is the same in the molecular world, and here the number 
of individuals involved is vast almost beyond conception, 
so that statistical prophecies are very reliable. However, 
what may be called the " individuality of molecular 
activities" is of great importance in modern physics. 

The " proof" of the hypothesis of molecular motion 
to which we have alluded lies in the remarkable corre- 
spondence which exists between the results of the kinetic 
theory and the facts of nature as determined by experi- 
ment, a correspondence which applies both to the statis- 
tical and to the individual behavior of the molecules. 


On the kinetic theory, see W. P. Boynton's " Application of the 
Kinetic Theory to Gases, Vapors, Pure Liquids, and the Theory 
of Solutions," 1904. 

Also W. Nernst's "Theoretical Chemistry" (1911), Book H, 
Chapter n, pp. 197-249. The latter account is perhaps the simpler 
of the two, and also the more empirical. 

Section 12 

The Molecular Counterpart of " Temperature" The 
exact temperature of bodies is not directly proportional 
to the speed at which then* molecules move, but rather 
depends upon the average energy of molecular motion. 



It is a familiar fact that the energy, or " kinetic energy," 
of a moving body depends not only upon the speed at 
which it is travelling but also upon its weight or "mass," 
or if we consider only bodies made up of the same sub- 
stance upon its size. Other things being equal, the 
larger a body is the more work must be done to set it 
in motion or to stop it, once it is moving. It is found by 
calculation from the laws of motion and by experiment, 
that the energy of motion, or kinetic energy, of any body 
is proportional to its mass and to the " square" of its 

This measure of energy of motion applies to molecules 
as well as to visible bodies, and so we must say that if 
the temperature of a piece of matter is proportional to 
the average, kinetic energy of its molecules its tempera- 
ture is proportional to the average square of the speed of 
these molecules, so that as their speed increases the 
corresponding temperature increases much more rapidly 
in proportion. From these considerations, also, it fol- 
lows that if we have a number of substances at the same 
temperature the speeds of their respective molecules will 
on the average, be less the larger (more massive) these 
molecules are. This must follow because at the same 
temperature the average energy must be the same re- 
gardless of the weight of the molecules, and this can 
only be true if the heavier molecules are moving at lower 
speeds than are the others. 

When a large number of atoms or molecules of differ- 
ent species and weights are mixed together the average 
energy of each species will be the same as that of any 
other species. It seems reasonable that this should be 
the case, for if the average energies were not equal in 
this way, the different substances making up the mix- 
ture would be at different temperatures, which is incon- 



sistent with the well-known fact that all bodies which 
are in close contact tend to come to the same tempera- 
ture. As we shall see later (Section 21), even when the 
vibrating particles are so large as to be visible under the 
microscope, their average energy of motion is approxi- 
mately the same as that of the very much smaller 
molecules. This is one aspect of what is known as the 
principle of "the equipariition of energy." 

Actual Molecular Speeds. We have seen in our pre- 
vious discussion that atoms and molecules differ widely 
in weight, and since weight and mass are proportional, 
it follows from what has been said above about the equal- 
ity of molecular energies of different substances at the 
same temperature, that the speeds of different molecules 
and atoms under these conditions will differ a great deal. 
The average speed of a hydrogen gas molecule at a tem- 
perature corresponding with the freezing point of water 
is about one and one-eighth miles a second, that of the 
mercury vapor molecule at the same temperature is 
about one-tenth of this, or about six hundred feet a 
second. The mercury molecule weighs just one hundred 
times as much as the hydrogen molecule. For the same 
temperature, molecular speeds vary as the square-roots 
of the molecular masses. Even the mercury molecule 
moves at the tremendous speed of four hundred miles 
an hour. Vast as this may seem, it is as nothing com- 
pared with the speeds which are attained by molecules 
and especially by electrons under other conditions which 
we are to consider at another point hi our discussion 
(see Section 25, below). 

The reader should bear in mind the fact that it is the 
average energy of motion which is constant at a given 
temperature. The speeds of the individual molecules 
differ enormously, but the variations in one direction 



balance those in the other so that from the statistical 
point of view which we have explained in Section 11 
there is constancy. 


For a detailed and simple account of the relations of molecular 
speeds in the kinetic theory see Chapter III of Part I of O. E. 
Meyer's " The Kinetic Theory of Gases." A list of concrete values 
for the speeds of molecules of thirty-one different substances is 
given on pages 57-58 of the English translation of this work (1899). 
"Molecular and Atomic Energies" are discussed in Chapter V of 
Part I. 

A still simpler discussion will be found in O. D. Risteen's " Mole- 
cules and the Molecular Theory of Matter" (1895), Chapter II. 

Section 13 


The "Mean Free Path." Since the molecules of a gas 
are moving helter-skelter it cannot be expected that the 
distance passed over by individual molecules between 
" bounces" will always be the same. However, from 
what we have said hi Section 11 about the " statistical" 
nature of the happenings in the molecular world it might 
be anticipated that on the average this distance would be 
constant for the same gas under the same conditions. 
This turns out to be the case, and the distance in ques- 
tion is known in the kinetic theory as the "mean (or 
average) free path" of the molecule. 

The mean free path of a gas molecule under standard 
conditions is a very important characteristic of the gas 
on account of the fact that upon its magnitude depend 
many of the obvious properties of the gas in question. It 
is clear that, other things being equal, this distance will 
be greater the smaller the moving molecules and the fewer 



there are of them in a given volume. In other words the 
average uninterrupted motion of the molecules is greater 
the smaller their chances of collision. When the gas is 
compressed the mean free path is diminished, so that it 
varies in a direction opposed to that of the change in 

Properties Depending on "Mean Free Path." Among 
other measurable properties of gases which depend upon 
the magnitude of the mean free path of then- molecules 
are to be mentioned their natural rates of " diffusion" 
(see Section 14) and the ease with which they conduct 
electricity. Any phenomenon which depends upon the 
rate at which individual molecules can move from one 
point to another hi the gas body will also depend upon the 
size of the "mean free path." Electricity is conducted 
through gases by being carried along bodily on electrons 
or molecules which move under the influence of the 
electrical forces exerted by the dynamo or battery. The 
longer the mean free path, that is the fewer the obstacles 
which oppose the motion of the electrified molecules, the 
less the "resistance" which is offered to the passage of 
the current. This explains why it is that gases under 
low pressure such as exist in many so-called " vac- 
uum-tubes" conduct electricity so much better than 
do the same gases at " atmospheric pressure." 

For atmospheric gases under ordinary conditions the 
mean free path is about one one-millionth of an inch in 
length. In vacuum tubes for the same gases it may be 
greater than one ten-thousandth of an inch. These mag- 
nitudes may seem very small, but when we consider the 
fact that the average gas molecule is only one three- 
hundred-millionth of an inch in diameter we see that 
the free movements of the molecules may be relatively 
large. The mean free path in hydrogen gas is greater 



under the same conditions than that in, say, mercury 
vapor, for the reason that the molecules of the latter 
gas are larger than those of the former. Although the 
length of the mean free path varies inversely as the 
square of the diameter of the molecules the difference is 
not very great, owing to the relatively small difference 
in diameter of the two species of molecules. 

On account of the great speed of molecular motion, 
the time intervals between the successive impacts of gas 
molecules are very minute even in highly rarefied gases. 


Concerning "Molecular Free Paths and the Phenomena Condi- 
tioned by Them" consult Part n of Meyer's "The Kinetic Theory 
of Gases" (1899). 

Section 14 

One very commonly observed phenomenon which can 
be accounted for only in terms of the motion of molecules 
is that of diffusion. The nature of this process is best 
illustrated by the manner in which odors travel. When 
a bottle of some pungent liquid is uncorked in one corner 
of an apartment the molecules of the vapor, which is 
always present above any liquid, swarm out of the bottle 
and ultimately spread themselves like insects to all parts 
of the room. Certain of them strike the sensitive sur- 
face of the nostrils and produce distinctive sensations of 
smell. Of course this motion of odoriferous molecules 
is assisted by the presence of air currents, but these 
become effective only when diffusion is also possible. 

The motion of the molecules in diffusion can seldom 
resemble a "bee line," since no molecule can be ex- 
pected to travel across the average room without en- 


countering countless others in its course. Diffusion 
movements must, therefore, have " jagged" paths, 

Fig. 16 


These jagged lines are supposed to be the paths of two gas molecules 
which are followed for a short period of time. They show quite clearly what 
is meant by saying that the motion of such molecules is haphazard, but, 
although the paths are very far from "bee lines," they nevertheless rep- 
resent a progressive displacement of the molecules from their original 
positions. It is in this manner that gases diffuse. The paths represented 
above were of course not obtained by the observation of gas molecules, 
but they come from a closely related source, viz., the measurements of M. 
Perrin of the so-called Brownian movement of small particles suspended 
hi liquids seen under the microscope. As explained in the text the Brown- 
ian movement follows the same laws as that of molecules in a gas. 

somewhat like that of a flash of lightning (see Figure 
16). On account of the tremendous obstacles which 


Sec. 15] SOUND ;' /;, ; 

are opposed to the straightforward motion of the mole- 
cules, the diffusion of large quantities of a gas or vapor 
requires long periods of time. 

Diffusion would not occur at all if it were not for the 
heat motion of the molecules. 


On diffusion see Meyer's work already referred to, Chapter HI 
of Part H. 

An account of the physical phenomena of diffusion will be found 
in A. L. Kimball's "The Physical Properties of Gases" (1890), 
Chapter VI. 

Section 15 


From the modern point of view sound must be regarded 
as a molecular phenomenon. 

Sound is commonly considered to be a species of wave- 
motion which is set up in the air, or other material sub- 
stance, by the vibratory motion of the object which is 
" emitting the sound." Suppose, for example, that a 
tuning fork is struck. The prongs of the fork are set 
into rapid vibration and at each of their excursions they 
push violently against the air molecules which surround 
them. These molecules are thus thrust away from the 
fork and communicate their motion to outlying molecules 
which act in turn upon a third layer of molecules further 
still from the fork. When the molecules near the fork 
have conveyed the impulse to the outlying molecules 
they rebound, just as does a moving billiard-ball 
which encounters a stationary one under the right 

It can be seen that an impulse of this sort will travel 
out from the source just as a wave of motion passes along 
a row of standing dominoes, the first of which has been 
overturned. The gas molecules, however, unlike the 



dominoes, return to their original positions when the wave 
or impulse has passed, and hence are ready for the gen- 
eration of a second wave, which is occasioned by the 
second excursion of the prong of the tuning-fork. The 
result is that a series of " rarefactions and condensations" 
among the molecules travel out from the tuning-fork and 
when these impinge upon the ear they cause the sensa- 
tion of sound. 

When the prong of the tuning-fork sets the air mole- 
cules in motion it of course endows them with some 
of its energy, and it is this energy which is responsible 
for the stimulation of the ear. The radiation of heat 
energy from a hot body is different from the radiation of 
sound energy in several ways. In the first place heat 
energy may be lost by two distinct processes. A hot body 
may lose heat energy on account of the fact that the 
motions of its surface molecules set up similar motions 
in the molecules of surrounding bodies, or it may lose 
energy by the generation of "heat waves." These con- 
stitute true "radiant heat" and do not consist in the 
motion of molecules, as do sound waves, but rather are 
closely similar to light, that is electromagnetic waves. 


An excellent semi-popular work "On Sound," although now an 
old one, is John Tyndal's book of that name (1888). The first 
Chapter is especially pertinent. See also Franklin & MacNutt's 
"Light and Sound" (1909). 

Section 16 

It is a well-known fact that when a solid melts, and 
that when a liquid is vaporized, definite amounts of heat 
energy are absorbed, the so-called latent heats of fusion 
and of vaporization. The cause of this absorption of en- 



ergy can be explained in a general way by the molecular 

The Latent Heat of Fusion. We have said that the 
molecules of a true solid body are unable to alter their 
relative positions and hence vibrate ceaselessly about 
the same centers. In the light of recent studies of the 
structure of crystals, it seems probable that the mole- 
cules of a solid are also not free to rotate, and that in a 
single crystalline unit they all point in the same general 
direction. This fixity of the molecules is what gives the 
solid its rigidity, and we must suppose it to be due to 
the existence of definite forces of attraction existing be- 
tween them. 

Any addition to the heat of a body occurs in opposition 
to these affinities. Melting occurs when the energy of 
motion acquired by the molecules is sufficient to enable 
them to break away from the chains of attraction which 
bind them to their neighbors. When this happens, how- 
ever, the attraction necessarily reduces the initial speeds 
of the molecules, because they constantly tend to be 
dragged back to then* original positions. Since heat 
consists in the energy of molecular motion, this slowing 
down of the molecules as they escape from their neigh- 
bors must mean the disappearance or " absorption" of 
a certain amount of heat, and this is the latent heat of 
fusion which we have mentioned above. 

It is obvious that when the temperature of a body is 
near the melting point only a relatively small force should 
be required to distort it, since the forces which hold the 
molecules hi their places are very much weakened. This 
consideration explains the increased "malleability" and 
plasticity which is exhibited by many bodies at high 

When a liquid cools, the forces of attraction again 


come into play and as the molecules drop into their posi- 
tions these forces increase the velocity of their motion, 
so that the latent heat once more appears in active form. 

Surface Tension. The molecules of a liquid are free 
to move anywhere within the liquid but are, for the most 
part, held within its bounds by a force which may be 
appropriately designated as the " attraction of the mass." 
The fact that an attraction of this character exists is 
proven by the phenomenon of surface tension. All liquids 
behave as if they were surrounded by a very thin and 
tightly stretched skin, an effect which is due to the strong 
attraction exerted upon the surface molecules by those 
which are underneath. 

The Latent Heat of Vaporization. The evaporation 
of a liquid consists in the escape of certain of its mole- 
cules through this surface film. In order to escape in this 
way they must move at a velocity sufficient to enable them 
to overcome the inwardly directed force which exists 
at the surface. This means that, in general, only the 
fastest moving molecules can gain their freedom, and 
even they achieve it at a certain price, namely a reduc- 
tion of their speed. 

The action whereby the fastest moving molecules are 
constantly being removed from the liquid together with 
this reduction of speed necessarily involves a loss of heat 
energy. This loss or absorption of heat is called the 
latent heat of vaporization. Everyone is familiar with the 
truth that the evaporation of a liquid has a cooling effect, 
and this effect is to be attributed to the fact that evapora- 
tion involves the absorption of heat energy. 

When a vapor molecule returns to the liquid from which 
it sprang, its speed is increased as it passes through the 
surface. In this way the latent heat of vaporization is 
reconverted into energy of molecular motion. 




Concerning latent heats consult H. C. Jones* "The Elements 
of Physical Chemistry" (1902), pp. 104-106, 161-162, which, 
however, does not deal with the molecular explanation of the 
phenomena. This is discussed, mathematically, hi W. Nernst's 
"Theoretical Chemistry" (1911), pp. 236-238. An elaborate dis- 
cussion of surface tension phenomena will be found in the eleventh 
edition of the Encyclopaedia Britannica under "Capillarity." 

Section 17 

As the temperature of a liquid is increased, a point is 
finally reached at which the influences of separation due 
to the movement of its molecules just balance the forces 
of inter-molecular attraction. This is called the " critical 
point" of the liquid. 

Since the film of surface tension which surrounds a 
liquid is due to the activity of its internal forces of attrac- 
tion, this film vanishes at the critical point, thus obliter- 
ating the distinction between the liquid and its vapor. 
Just below the critical point the latent heat of vaporiza- 
tion is practically zero, on account of the absence of any 
effective forces of attraction to be overcome hi separating 
the molecules. Both the surface tension and the latent 
heat of vaporization decrease gradually as the tempera- 
ture rises. 

Under ordinary conditions the majority of liquids boil 
at temperatures which are far below their critical points. 

Since the atmosphere above a liquid exerts a confin- 
ing pressure upon it, it is impossible for vapor to form 
within the mass of the liquid until the pressure exerted by 
the vapor itself is greater than that of the atmosphere. 
When this point has been reached the production of vapor 
within the body of the liquid results in the formation of 



bubbles which rise to the surface and break, the familiar 
phenomenon of boiling. In accordance with this explana- 
tion it is easy to see why the boiling points of all liquids 
should be lowered by a decrease in the pressure of the 
surrounding atmosphere, and raised by an increase in 
the pressure. 


On critical and boiling points see A. D. Risteen's "Molecules 
and the Molecular Theory of Matter" (1895), pp. 80-84. 
Also Nernst's "Theoretical Chemistry" (1911), pp. 63-67. 

Section 18 

Boyle s Law. The relations which exist between 
the pressure which is exerted by a gas, its temperature, 
and its state of compression, i.e., its density, are very 
simple, and are at once accounted for by the molecular 
theory. The pressure acting upon a vessel which con- 
tains a gas must obviously become greater when the size 
of the vessel is decreased, because although the number 
of molecules remains constant, the frequency with which 
they strike the sides of the vessel must increase. This is 
the basis of the well-known law of Boyle, which states 
that the pressure exerted by a given quantity of gas is 
inversely proportional to the volume which it occupies. 
The law can in fact be derived mathematically by con- 
sidering the action of the moving molecules. 

Charles Law. When the temperature of a gas in- 
creases, the molecules move faster, and hence the pres- 
sure must increase also. It can be shown by a simple 
calculation that the pressure caused by the bombardment 
of the sides of the vessel by the flying molecules should 
be proportional to the average energy of this motion. This 



means that the pressure of a gas is directly proportional 
to its temperature measured in degrees above the " ab- 
solute zero": the law of Charles. 

" Absolute Zero." " Absolute zero" is defined as a 
point of temperature at which the molecules are motion- 
less and hence as a point at which the gas-pressure is also 
zero. By measurements upon gases we can find out how 
much their pressures decrease for each unit of tempera- 
ture, and if we then divide their pressure at any tem- 
perature by this amount, we shall learn the number of 
degrees which must be subtracted from the temperature 
in question in order to give us the absolute zero. This 
is the principle of Gay-Lussac. As should be expected, 
it turns out that the change in pressure for a given 
change in temperature is approximately the same pro- 
portion of the total pressure for all gases studied under 
the same conditions. 

The Principle of Avogadro. In Section 12 we have 
discussed the principle according to which all species of 
molecules at the same temperature have the same aver- 
age energy of motion, regardless of their other char- 
acteristics. If this principle is valid the pressure exerted 
by a given body of gas should be independent of the kind 
of molecules of which it is made up, and should depend, 
as we have stated above, merely upon the number of 
molecules of any sort whatever which are present. From 
this it follows that if the same vessel is filled successively 
with different kinds of gas at the same temperature and 
pressure, the same number of molecules will be present 
hi each case, or in other words: " equal volumes of all 
gases under the same conditions of temperature and 
pressure contain equal numbers of molecules," which 
is the famous principle of Avogadro. 

As we have already seen (Section 5, above), it follows 


from Avogadro's rule that equal volumes of similarly con- 
ditioned gases should have weights which are in the same 
proportion as the weights of their respective molecules. 
This is another consideration which affects the form of 
the so-called "gas law," a familiar formula which sum- 
marizes the relationships which we are now discussing. 

The Formula of van der Waals. It has been shown by 
experiment that the law of Boyle does not hold for high 
pressures and low temperatures. The reason for this 
we have already mentioned in Section 2. It is to be 
found in the fact that Boyle's law is calculated on the 
assumption that the molecules are geometrical points, 
whereas in reality they have a volume of their own, a 
fact which must affect the ease with which the gas is 
compressed. Moreover the gas molecules exert an at- 
traction upon each other which tends to make compres- 
sion easier at high than at low pressures. The influence 
of these factors has been summed up in the very accurate 
gas formula of van der Waals. 

As pointed out in Section 19, below, the above con- 
siderations apply, at least approximately, to substances 
in the dissolved, as well as in the gaseous state. 


Concerning the laws of gases consult A. D. Risteen's " Mole- 
cules and the Molecular Theory," pp. 40-58; W. Nernst's 
11 Theoretical Chemistry" (1911), pp. 198-201, or G. Senter's 
" Outline of Physical Chemistry" (1908), Chapter H. 

Section 19 

When a substance, such as sugar, is dissolved in (say) 
water, its molecules are separated from each other and 
wander about among the water molecules. If we neglect 

[ 108 ] 


the presence of the latter we may regard the state of the 
sugar as essentially that of a gas having a density corre- 
sponding to the concentration of the sugar in the water. 

Now there is an arrangement by means of which it can 
be shown that a dissolved substance actually does behave 
like a gas. Suppose that some sugar solution is placed in 
a balloon made of a membrane through which water can 
pass without difficulty, but which is impenetrable to the 
molecules of sugar. If we now place this balloon in a 
glass of water it will tend to expand and may even burst. 
This effect is due to the fact that the sugar molecules hi 
the course of then* heat vibrations strike the sides of 
the balloon, and being unable to pass through it as the 
water molecules do, they tend by their impact to force 
it outwards. 

General considerations lead us to believe that the laws 
of this so-called " osmotic pressure" should be the same 
as those of gases, and this is shown by measurements to 
be approximately the case. 


A popular discussion of the phenomena of osmotic pressure 
appears in W. C. D. Whetham's "The Recent Development of 
Physical Science" (1904), pp. 104-124. For more advanced con- 
siderations, see H. C. Jones' "The Elements of Physical 
Chemistry" (1902), pp. 179-199. 

Section 20 

Evidently, if the molecular theory is true, gases should 
be better conductors of heat, per unit of mass, than are 
liquids, since their constituent molecules are more free 
to change their positions, thus permitting a more rapid 
mixing of the fast and the slow. For the same reason 



liquids should be better conductors than solids. These 
expectations appear to be borne out by the facts of nature. 
There seems to be an exception to the rule, however, 
in the case of metals, which are the best conductors of 
heat known. This apparent exception is explained, how- 
ever, by the fact that metals contain vast numbers of 
u free electrons," particles so small that they can travel 
about among the molecules of the metal almost as if 
they were in unobstructed space. Indeed, the metal may 
be said to contain negative electricity in gaseous form. 
These minute particles partake of the heat vibration of 
the molecules, and by their very rapid motion quickly 
bring the temperature of all parts of a metallic body to a 
uniform level. Metals are as good conductors as the 
lightest gases because the mass of an electron is exceed- 
ingly small, even as compared with that of the hydrogen 
atom, and hence, in accordance with the equipartition 
principle (see Section 12) must move faster at the same 
temperature. Besides this, it is probable that electrons 
can pass through the body of an atom without being 


On the conduction of heat see Meyer's "The Kinetic Theory 
of Gases," Part H, Chapter IX (trans. 1899). 

Section 21 

Perrin's Experiments. The work upon the physics of 
the Brownian movement is still new. Credit for the ex- 
perimental side of the investigation belongs largely to the 
French scientist Jean Perrin, whose methods of measur- 
ing the movements are exceedingly ingenious. 

The Brownian particles which were employed by 
Perrin were produced by the precipitation of alcoholic 



solutions of various gums by pouring these solutions into 
water. They varied in size from about one twenty-five- 
thousandth to one two-hundred-and-fiftieth of an inch in 
diameter. Their motions were measured by direct ob- 
servation under the microscope, and also indirectly by 
means of certain calculations. 

Perrin was able to show that these minute drops of 
gamboge and other gums, when made into emulsions with 
water, behave in every way like the molecules of a gas 
of enormous molecular weight. This applies to such 
characteristic processes as average speed, rates of dif- 
fusion, pressure, etc. 

Verification of Equipartition of Energy. One very 
interesting outcome of Perrin's work lies in its remark- 
able verification of the principle that the average energy 
of vibration of any species of particle depends only on the 
temperature of the mass of matter considered and not 
on the weight or size of the particles themselves (Sec- 
tion 12). Some of the particles which he studied were 
many thousand times as large and heavy as the heaviest 
known atom, and yet their average energy appeared to 
be substantially identical with that characteristic of all 
atoms or molecules at the temperature under observa- 
tion. He was able to prove, moreover, that the particles 
have an average energy of rotation which is the same as 
that of their translatory movements, a result in harmony 
with the demands of theory. 

Other investigations have shown that the Brownian 
movement in gases follows the same laws as those which 
hold among particles suspended in liquids. 


Perrin's own account of his researches on the Brownian move- 
ment will be found in "The Brownian Movement and Molecular 
Reality," translated by F. Soddy (London, 1910). For the most 
part the book is not difficult reading. 



Section 22 

The Crystal as a Unit of Structure. The statement 
that solid bodies are characterized by an orderly arrange- 
ment of their molecules implies that all solids are crys- 
talline. This implication is probably correct hi spite of 
the fact that chemistry distinguishes between crystalline 
and the so-called " amorphous" solids. Amorphous, or 
formless, bodies may be regarded as crystalline bodies 
in which the crystals are very small. 1 Strong reasons 
exist for believing that this holds even for the so-called 
"colloidal" substances, which are usually contrasted 
with "crystalloids." 

If this is the true view the arrangement of the mole- 
cules in the entire mass cannot be quite orderly, but it 
is probably permanent. When solids are broken the 
surfaces of fracture generally coincide with the surfaces 
of the crystals of which they are composed. It would not 
be wrong to regard an ordinary solid body as a closely 
packed mass of smaller bodies, the individual crystals, 
which alone represent the characteristic form of a solid. 
The crystal thus becomes the unit of solid matter next 
in order above the molecule. However, there are some 
perfectly definite crystals which seem to be decompos- 
able into smaller, similar, crystals without limit. Such 
a substance, for example, is mica, which crystallizes hi 
sheets, but no matter how thin a sheet of mica may be, 

1 As elsewhere stated, the recently discovered " liquid crystals " 
introduce a somewhat mysterious element into these considera- 
tions. It seems probable that in the last analysis liquid crystals 
will be found to depend on a somewhat different set of forces 
than do crystals of solids. 



it is always theoretically possible to split it into two 
thinner sheets, provided, of course, that the first one is 
not of molecular thinness. It seems to be characteristic 
of the molecules which compose the mica to arrange 
themselves in geometrical planes. 

Crystal Structure as Studied by X Rays. Very re- 
cently it has been found that X rays, when reflected from 
a crystal surface, are broken up into a pattern the nature 
of which varies with the crystal employed. The principle 
in accordance with which this pattern is formed is well 
known, being that of optical diffraction or interference 
so that from the character of the pattern it is possible 
to deduce the arrangement of the molecules within the 
crystal. It appears from these studies, the basis of which 
will be further discussed in Section 55, below, that the 
unit of crystalline structure, from a geometrical point 
of view, at least is not the molecule, but the atom. 
Crystal form appears to result from an extension of the 
same architectural principles upon which the molecule 
is built. 

The cubical crystal of potassium chloride, for example, 
seems to be made up of a rectangular lattice-work of 
alternate potassium and chlorine atoms, placed at equal 
distances from one another. It is natural that there 
should be a close similarity between the external shape 
of the crystal and that of the spatial configuration of its 
component atoms, but it is not always possible to infer 
the latter from the former. For instance, the crystal of 
potassium bromide, which is externally similar to that 
of the chloride, appears to have atoms not only at the 
corners of a simple cubical lattice-work, but also in the 
centers of all of the cube faces. Sodium chloride, or 
common salt, another cubical crystal, has an even more 
complex structure. 



Stages Between Solid and Liquid: Liquid Crystals. 
Of course all conceivable stages exist between the solid 
and the liquid states. It would be difficult, for example, 
to say whether such a substance as asphalt under certain 
conditions of temperature is a solid or a liquid, and ex- 
periments have shown that even very hard and brittle 
substances like the crystals of common salt exhibit defi- 
nite evidence of vaporization, a process usually ascribed 
only to liquids. 

Liquid crystals probably depend upon the definite 
relative arrangement of molecules which may neverthe- 
less alter then* absolute position. Just as hi the living 
organism, the actual matter changes as time goes on, 
although the form remains practically constant. In other 
words, it is not impossible to conceive a combination of 
orderly arrangement, such as is demanded by the crystal- 
line state, with relative freedom of translatory movement 
of the molecules among themselves, which seems to 
characterize the liquid state. In ideal solids, however, 
whatever the arrangement, this movement cannot oc- 
cur, and motion of the molecules must be exclusively 

It is to be expected that the new X ray method of 
studying crystalline structure will eventually clear up 
most of these mysteries, and at the same time throw light 
upon the very closely related problem of the constitution 
of the molecule. As yet, only a few simple crystals have 
been analyzed by this means. 


On the "Molecular Theory of Solids" see Risteen's " Mole- 
cules and the Molecular Theory of Matter," Chapter IV. 

A German work on liquid crystals is: "Die Neue Welt der 
Flussigen Kristalle," by O. Lehmann (1911). 



On the analysis of crystals by means of X rays, see G. W. C. 
Kaye's "Xrays" (1914), pp. 168-204; and W. H. and W. L. 
Bragg's "X Rays and Crystal Structure" (1915). 

Section 23 


The "Distribution Curve" of Molecular Speeds. In 
Section 11 it is said that the behavior of molecules 
can be studied satisfactorily only by the use of the sta- 
tistical method. Even at a constant temperature all of 
the molecules do not move at the same speed; it is the 
average speed which is constant. However, because of 
the enormous number of molecules which are contained 
in any body which we may consider, it is possible to make 
true statistical statements which give us more detailed 
information about the state of affairs hi the body than 
does the mere knowledge of the average velocity of the 

Some of the molecules of a body move faster than the 
average and others move more slowly, but on account 
of the vast number which are present and the consequent 
great frequency of their collisions there exists a constant 
levelling tendency, a continuous redistribution of energy, 
which tends to make them all approximate the average 
speed. Theoretical considerations show that in a chaos 
of molecules such as is contemplated by the kinetic theory 
there must be far more molecules moving at approximately 
the average speed than at any other speed, and that the more 
the speed of a molecule departs from the average the 
fewer of its kind there must be. This principle is often 
spoken of as the "a law of distribution of molecular 
speeds," and it is of the utmost importance in the study 



[Sec. 23 

of heat and allied phenomena. As shown in Figure 17, 
it is mathematically similar to the well-known " curve 
of chance " with which the reader may be familiar. 

The Laws of "Vapor Pressure" Evaporation, it has 
been explained, consists in the escape from the surface 

Fig. 17 


This curve shows geometrically the relative number of molecules mov- 
ing at speeds which differ more or less from the average speed. Relative 
speed is measured from left to right along the horizontal line and relative 
number along the vertical line. A is the point corresponding with the 
" average energy " of all of the molecules. It is evident from the dia- 
gram that there are more molecules moving at approximately this speed 
than at any other, and that the more any (approximate) speed differs from 
the average the fewer will be the molecules moving at this speed. This 
curve is a special case of the so-called "curve of error" which represents 
a law of the utmost importance in modern physics. 

of a liquid of certain molecules which move faster than 
the average. According to the above law of the dis- 
tribution of molecular velocities, as the temperature of 
the liquid is increased the number of molecules which 
escape should increase also, and the exact nature of this 
increase can be predicted from the law. 
The vapor above a liquid is a gas, and hence exerts a 


pressure upon the bodies immersed in it. This pressure 
must follow the ordinary gas laws which we have dis- 
cussed in Section 18. Since according to these laws the 
pressure is proportional to the number of molecules 
present in a given volume, and since this number increases 
with the temperature of the liquid, the "vapor pressure," 
as it is called, should increase with the temperature also, 
and in a way harmonious with the law of distribution of 
molecular speeds. 

Empirical measurements show that the actual rise of 
the vapor pressure of all liquids is quite closely in accord- 
ance with the theoretically deduced law. Everybody is 
acquainted with the general fact that liquids evaporate 
faster the hotter they are. 

It might at first be thought that the vapor escaping from 
a liquid would be at a higher temperature than the liquid 
itself, because only the fast-moving molecules are able to 
pass through the surface. However, as we have indicated 
in Section 16, the speeds of all of these molecules are 
slowed down under the influence of the film of surface 
tension. It so happens that this decrease, which stands 
for the absorption of the latent heat of vaporization of 
the substance, is of exactly the right magnitude to reduce 
the average speed of the escaping molecules so that the 
temperature of the vapor is the same as that of the liquid 
from which it rises. This can be shown theoretically by 
a consideration of the law of distribution of molecular 


On the "law of distribution" refer to Meyer's work, Chapter 
in of Part I (see Note 12). 

A mathematical discussion of vaporization will be found in 
Chapter VII of H. P. Boynton's "Applications of the Kinetic The- 
ory, etc." (1904). 



Section 24 

The total heat energy of any body is the sum of the 
energies of motion of all of its molecules. It can be esti- 
mated approximately by multiplying the average energy 
of the molecules by the total number of molecules in 
the body. 

When the temperature of a given weight, say one 
ounce, of a substance is increased one degree, a definite 
amount of energy has to be added to it in the form of heat. 
If this amount of energy be divided by the amount re- 
quired to bring about the same change in an equal weight 
of water, the quotient is the "specific heat" of the first 

Atomic Heats. Measurements have shown that the 
specific heats of different substances vary quite widely, 
but it was discovered by the French investigators, 
Du Long and Petit, that if the specific heat of any ele- 
mentary substance in the solid state be multiplied by 
its atomic weighty the resulting product is approximately 
6, no matter what element is taken. The reason for 
this striking fact is not far to seek. It is found in the 
principle that the average energy of motion of any group 
of molecules is independent of their species (see Section 
12). If all atoms at the same temperature have the same 
energy of motion, then the energy which must be added 
to their motion to produce a change in temperature must 
be independent of their species and hence of the nature 
of the substance involved. Multiplying the specific heat 
of an element by the atomic weight reduces the meas- 
ure of heat capacity to terms of number of atoms alone, 
eliminating the factor of weight. 



To raise the temperature of a gas it is only necessary to 
increase the energy of motion of its molecules, but the 
molecules of solids and liquids are bound together by 
strong forces of attraction, and when their vibrations are 
increased, an amount of energy must be utilized in over- 
coming these forces which is equal to that which enters 
into the increased motion. Accordingly, the specific heat 
of a substance in the solid or liquid form should be about 
twice that in the gaseous form, and this is found to be 
the case in nature. 

Now the considerations which apply to elementary 
substances apply also to compounds, although with less 
accuracy. When the specific heat of a compound is mul- 
tiplied by its molecular weight, that is by the sum of the 
weights of its contained atoms, the product does not 
vary a great deal from 6, if the substance is in the solid 
form, or 3, if it is in the gaseous form. However, the 
variations which do occur are sufficient to require explana- 
tion, and this must be given in terms of the " force con- 
stitution" of the molecule of which we have spoken at 
some length in Section 8. In general, the stronger the 
internal forces of the compound the higher will be its 
specific heat, since a large amount of energy will be 
absorbed in setting the parts of its molecules into relative 
vibration, if the attractions which hold these parts to- 
gether are powerful. It is necessary that the atoms 
should vibrate within the molecule as well as with the 
molecule as a whole, and the average energy of this 
internal vibration should be equal to that of the grosser 
molecular movement. 

It has recently been discovered that the specific heats 
of all substances decrease very rapidly at very low tem- 
peratures, so that at absolute zero they would probably 
themselves be zero. The exact meaning of this strange 



fact is not yet clear, but it seems to be related with the 
newly suspected atomic nature of radiant, and perhaps 
all, energy. We shall discuss this matter briefly in Sec- 
tion 54. 


The specific heat of gases is discussed on pp. 63-74, of Jones' 
" Elements of Physical Chemistry" (1902), liquids on pages 106- 
110 and that of solids on pp. 162-166. 

Consult also James Walker's "Introduction to Physical Chem- 
istry" (1901), Chapter V. 

On the changes in specific heats which occur at low temperature, 
see W. Nernst's "Theoretical Chemistry" (1911), pp. 710-716. 

Section 25 


Thomson s Determination of the Electronic Mass and 
Charge. The electron was discovered by J. J. Thomson 
as the result of a series of epoch-making and very ingen- 
ious experiments. When an electrical discharge passes 
through a tube from which the air has been partly ex- 
hausted it consists, hi part, of a beam of rays which 
seem to be emitted from the negative pole, or " cathode," 
of the battery or induction coil. (See Figure 18.) 
Thomson showed that these so-called "cathode rays" 
are made up of very minute bodies nearly two thousand 
times lighter than hydrogen atoms, and moving at a 
speed which varies with conditions but which is in gen- 
eral about one-tenth that of light, or about twenty 
thousand miles a second. 

Thomson did not determine the size of the electron 
directly but only its weight, or more strictly speaking its 
mass. This he was able to do by use of the well-known 
principle that the more massive a body is, and the higher 



its velocity, the greater is the resistance which it offers 
to change in its state of motion. It was found that the 
cathode rays could be bent by the action of a magnet, a 
fact which showed them to bear electrical charges, and 
by measuring the magnitude of this bend as compared 
with the strength of the magnet employed, a basis was 
provided for the calculation of the mass of the moving 

Fig. 18 


The cathode rays are emitted from the plated and travel away from it 
In straight lines, a fact which is shown by the character of the shadow, 
C, cast by the metal cross, B. This shadow appears in the glow which is 
produced where the rays strike the end of the tube. 

particles. (See Figure 19.) Measurement of the bend 
of the rays under the influence of magnetism alone did 
not permit Thomson to separate the effect of the speed at 
which the particles were travelling from that due to their 
mass, but by studying the bend which occurred when 
electrical as well as magnetic forces were brought to bear 
upon the rays, he was able to obtain a measure which 
depends upon the mass and not upon the velocity of the 



[Sec. 25 

However, it unfortunately happened that these meas- 
ures were not independent of the electrical charge borne 

by the single particles, 
and consequently he was 
obliged to devise a 
method for the determi- 
nation of this charge. 
The method which he 
actually used consisted 
in a simple measurement 
of the total amount of 
electricity carried by a 
large number of the 
particles, followed by a 
determination of the 
number itself. From 
these two measurements 
the quantity of electricity 
carried by one particle 
could obviously be calcu- 
Thomson's procedure 

It is seen that after the rays enter the large f r finding the number of 
bulb A they move along a circular path. particles Corresponding 

to a known charge was 
exceedingly clever. On 
account of its charge each 
of the particles is a center 
of forces of attraction, 
so that if they exist hi 
an atmosphere over-sat- 
urated with moisture, 
this moisture tends to condense about the individual 
particles, each of the latter thus becoming the nucleus 


Fig. 19 


This drawing represents a cross-section 
of a tube which was employed by J. J. 
Thomson in the study of the cathode rays. 

This is due to the presence of a magnet 
which is placed outside of the bulb in such 
a way that the lines of magnetic force 
between the opposite poles of the magnet 
are perpendicular to the path of the rays. 
The magnet is omitted from the drawing 
so that the path of the rays can be clearly 
shown. However, if it were supposed to be 
really absent it would be necessary to 
represent the rays as impinging on the 
bulb at C instead of at B, which is an 
electrical condenser for collecting the 
charge carried by the rays. 

I I 


of a small drop of water. The size of the drops of water 
thus formed can be determined by the rate at which the 
fog which they compose settles under the pull of gravity. 
Since it is easy to measure the total amount of water 
which condenses, the number of droplets in the fog can 
be calculated from a knowledge of their individual size. 
Since each droplet corresponds to a single electrical 
particle the number of droplets gives a measure of the 
number of such particles which are present, and hence 
permits the calculation of the charge which they indi- 
vidually bear. 

Knowing the amount of electricity carried by each 
particle in the cathode rays, it is possible to separate the 
effect of the charge from that of the mass, and hence to 
ascertain the magnitude of the latter. The most refined 
measurements of this sort show that the cathode ray 
particles, which are now called electrons, have a mass 
which is about one eighteen-hundredth that of the lightest 
known atom, that of hydrogen. 

Sources of electrons other than the cathode rays are 
now available, and the nature of electrons has been satis- 
factorily proved to be independent of their source. 

The Size and Shape of the Electron. The size of the 
electrons is calculated by use of the fundamental laws 
of electrical action. These laws imply that electrical 
charges, even when free from all matter in the ordinary 
sense of the word, possess one of the most characteristic 
properties of matter, viz., mass or inertia. The laws state, 
furthermore, a definite relationship between the charge 
of an electrical particle, its volume, and its mass, such 
that for a given charge the mass increases as the volume 
decreases. Since we know the mass and the charge of 
the electron from the measurements described above, it 
is possible to calculate the volume. This calculation is 



based upon the assumption that the electron is made up 
of pure negative electricity and of nothing else, i.e., that 
it contains no "matter," as distinguished from electricity. 
Although it is difficult to give a direct justification of this 
assumption it is nevertheless in harmony with all of the 
analogies of the situation, and is contradicted by none of 
the facts. Studies in radio-activity have shown that 
electrons can pass straight through considerable thick- 
nesses of solid substances and, indeed, through the atoms 
themselves. This clearly suggests that the electrons are 
very minute, as is indicated, also, by the calculations. 

The symmetry of structure of the electron seems to 
be borne witness to by certain measurements regarding 
the manner hi which its mass changes with its velocity. 
It follows from the fundamental electrical laws mentioned 
above that the electronic mass will increase at high 
speeds in a way which depends in its details upon the 
shape and also upon the internal structure of the electron. 
By calculating the masses which should be effective at 
different speeds for various probable types of electronic 
constitution, and then comparing these results with actual 
measurements we can obtain some idea as to the real shape 
and structure of the electron. At present the electron is 
believed to be spherical at low speeds, but it is thought 
that it becomes more or less flattened hi the direction of 
motion when it moves at speeds approaching that of light. 


A considerable number of popular discussions of the electron 
and its measurement are obtainable. Among these may be men- 
tioned the following: 

R. K. Duncan's "The New Knowledge" (1908), Part 3, Chapters 
III to X, inclusive. 

Sir Oliver Lodge's "Electrons" (1907), Chapters HI-XIV 



Harry C. Jones' "The Electrical Nature of Matter and Radio- 
Activity" (1906), Chapters I-III inclusive. 

E. E. Fournier D'Albe's "The Electron Theory" (1906), Chap- 
ter XI. 

Section 26 


The meaning of the statement that "most of the 
phenomena hi nature are due, hi the last analysis, to 
electrical attractions and repulsions," will become clear 
to the reader as he proceeds. Part of its significance 
can be grasped at the present stage of the discussion if 
one remembers what has been said hi Section 8 about 
the dependence of the properties of substances upon 
the nature of their internal forces. There is now little 
doubt that these forces are electrical. Also, chemical 
action and all electrical phenomena clearly involve the 
agency of electrical forces. When we come to study the 
question of the constitution of the atom we shall see that 
the phenomena of radio-activity, and the emission and 
absorption of light, have an electrical origin. 


See Sir Oliver Lodge's "Electrons" (1907), Chapter XVI. 

Section 27 


How Ions are Produced. When an electron is taken 
from or added to a previously neutral atom or molecule 
the charged particle which is thus formed is called an 
"ion" and the process is that of " ionization." Various 
means of ionization are known. The collision of molecules 



in the course of their heat vibration may sometimes be 
sufficiently violent to knock electrons out of the molecules. 
A more effective process of a similar nature, however, 
lies in the bombardment of a gas with flying electrons or 
ions, which on account of their speed and the electrical 
forces which they exert upon the electrons within the 
gas molecules are able in many cases to bring about a 
separation of the two. C. T. R. Wilson, by bringing about 
the condensation of moisture on the ions which are 
formed, has been able to obtain accurate photographs of 
the path of the flying a particles from radium through a 
gas. A powerful electrical field such as that which' 
exists between the terminals of a sparking induction 
coil will cause ionization. Light (including X rays), 
being a form of electrical energy, can also separate elec- 
trons from the atoms with which they are combined. 
Ionization is likewise a common accompaniment of chem- 
ical action, and occurs in many chemical solutions, a fact 
later to be considered in greater detail (see Section 29). 

A definite amount of energy is required to force an elec- 
tron out of any atom, an amount which varies only slightly 
with the nature of the atom. The differences which exist, 
however, are sufficiently constant to constitute charac- 
teristic properties of the elements. The energy of ioniza- 
tion of a substance can be estimated from the intensity 
of the electrical field needed to just produce ionization. 

The Interactions of Ions and Electrons. As explained 
in Part I an " uncharged " atom contains a certain number 
of electrons and also positive electricity, enough to neu- 
tralize exactly their negative charges. If an electron is 
added to the atom from the outside there will be more 
negative electricity than positive and the atom will have 
a " negative charge"; whereas if an electron is taken away 
from it there will be more positive than negative elec- 


Sec. 27] 


tricity and the atom will have a "positive charge." 
(7) in Figure 20 shows two uncharged atoms, (4) two 
negatively charged ones, and (5) two positively charged 







Fig. 20 



These diagrams represent in a symbolic way the forces which operate 
between aggregates of electrical charges, of various degrees of complica- 
tion. The diagrams are explained in the text. 

ones. It must be borne carefully in mind that these are 
not pictures of atoms. They are merely symbolic draw- 
ings, the black dots representing electrons, and the 



"plus sign" representing the positive charge which is 
inseparable from the atom. 

It follows of course from the laws of electricity, as re- 
called to the reader in Chapter V under the heading, 
"The Two Electricities," that the following statements 
are true : 

(a) Two electrons repel each other [see (1), Figure 20]. 

(b) An electron is repelled by a negatively charged 
atom (i.e., one which has one electron too many for 
neutrality (2). 

(c) An electron is attracted toward a positively charged 
atom (i.e., one which has one electron too few for neu- 
trality) (3). 

( d) Two negatively charged atoms repel each other (4). 

(e) Two positively charged atoms repel each other (5). 
(/) A positively charged atom and a negatively charged 

atom attract each other (6). 

There are also two attractions of a different kind, one 
of which is already familiar to the reader, which do not 
follow obviously from the fundamental electrical laws. 
These are : 

(g) All atoms attract each other, even when they are 
neutral (7). This is the familiar attraction considered 
hi Section 10. It explains the cohesion of solids and liquids 
in spite of the violent heat vibration to which their atoms 
and molecules are subject. This attraction, unlike the 
common "electrical" attraction (/), is effective only when 
the two atoms are very near each other. 

(h) All uncharged atoms attract electrons (8). This 
force, like (g), is only effective when the electron is very 
near the atom. When it is near, however, the force be- 
comes very great. At greater distances it is very much 
weaker than the familiar "electrical" forces before 




Concerning ionization, consult "The Electron Theory" by E. E. 
Fouraier d'Albe, Chapter IV; " Electrons," by Sir Oliver Lodge, 
Chapter VII; "Modern Theory of Physical Phenomena" by 
Augusto Righi (1904), Chapter IV. 

Section 28 


The "Hall Effect." It follows from a fundamental 
law of electrical science that when a moving electron 
comes under the influence of a magnet its normally 
straight-line path will be changed to a curve. Hence if 
the conduction of electricity through solids actually con- 
sists in the bodily motion of electrons it should be possi- 
ble to alter the direction of an electric current in a wire 
by bringing a sufficiently powerful magnet near it. Ex- 
periment shows that this can be done, the phenomenon 
being commonly known as the " Hall effect." There are 
certain stubborn difficulties in connection with the ex- 
planation of the Hall effect, because the changes are some- 
times in one direction and sometimes in the other, but 
it seems highly probable that the phenomenon is due to 
electron deflection. 

The Nature of Electrical Resistance. Different metals 
and substances in general vary widely in their so- 
called electrical conductivity, or to put it the other way 
round, they offer varying degrees of "resistance" to the 
passage of the electrical current. There are various 
factors which determine the electrical conductivity or 
resistance of substances. In the first place, it should be 
clear that the more electrons a substance contains in a 
given volume the more electrons will move forward when 



an electrical force is applied to it. The "current" or 
" amperage" is merely the amount of electricity which 
flows through a certain portion of the wire in a given time, 
or, hi other words, the number of electrons which pass 
any fixed boundary. So it is obvious that the more 
electrons there are free to move the greater will be the 
current under a given electromotive force or " voltage," 
and therefore the higher the conductivity of the sub- 
stance or the lower its resistance. Substances such as 
hard rubber and porcelain contain almost no free elec- 
trons and hence are what we call "non-conductors" or 
"insulators." Metals like copper and silver contain a 
large number of free electrons and accordingly are 
"good conductors." 

Electrical and Thermal Conductivity. In Section 20 
the fact is mentioned that the extraordinarily good heat 
conductivity of metals is accounted for hi terms of the 
free electrons which they contain. If this is the true 
explanation it can be shown to follow that, other things 
equal, those metals which contain the largest number of 
free electrons will be the best heat conductors. But such 
metals will also be the best conductors of electricity, and 
hence it would appear that some sort of proportionality 
should exist between the power of a substance to conduct 
heat and its power to conduct electricity. Accurate meas- 
urements and calculations show that a relationship of 
this kind holds in nature, and that its quantitative char- 
acter is hi remarkable accord with the assumptions of the 
electronic and molecular theories. 

Besides the number of free electrons in a unit volume 
of a substance there are other factors which must influ- 
ence its conductivity. One of these is the "mean free 
path" of the electrons among the molecules of the sub- 
stance (see Section 13). Other things being equal, the 



farther an electron can move without striking an atom or 
another electron the better the substance will conduct 
both electricity and heat. 

It is interesting to note the fact that the direction of 
movement of the electrons in a wire is opposite to the 
so-called direction of the current, for the reason that the 
latter is what would be the line of motion of positive 
electricity if any were moving. The electrons, it will be 
remembered, are negative. When they move oppositely 
to positive particles the two produce identical magnetic 
effects. If electrons had been known when the termi- 
nology was developed, the conventional direction of the 
current would probably be the reverse of that now in 


The following references are to simple discussions of the theory 
of electrical conduction in solids: 

E. E. Fournier d'Albe's "The Electron Theory," Chapter IV, 
Section 4. 

Sir Oliver Lodge's "Electrons," Chapter X. 

D. F. Comstock: "The Modern Theory of Electric Conduc- 
tion," in the "Transactions of the American Electro-Chemical 
Society" (1912), Volume XXI, pp. 41-48. 

A somewhat mathematical and more detailed account is given 
by Sir J. J. Thomson in his " Corpuscular Theory of Matter " (1907), 
Chapters IV and V. 

Section 29 

Conduction by Ions; Electrolysis. Metals are not the 
only substances which are good conductors of electricity. 
It is a well-known fact that many solutions are excellent 
conductors, and in this case the conduction involves the 
motion not of free electrons but of charged atoms or 
"ions." Some of these moving atoms are negatively 



charged, that is, bear electrons in excess of their normal 
number, while others are positive and have lost part of 
their regular complement of electrons. The current 
through the liquid consists of negative atoms or ions 
moving in one direction and of positive atoms or ions 
moving in the opposite direction. Both of these lose 
then* charges when they come into contact with the 
"electrodes" by which the current enters and leaves 
the solution, the former at the positive pole and the 
latter at the negative pole. This means that atoms of 
the positively charged substances will collect about the 
negative pole while those of the negatively charged kind 
will segregate about the positive electrode. This process 
is commonly called electrolysis, and will be further dis- 
cussed at another point. For this type of conduction it is 
necessary for the liquid to contain ions. (See Section 27.) 
The conduction of electricity through gases also de- 
pends upon the presence of ions. Free electrons, however, 
are often present and active. To the study of phenomena 
connected with the passage of electricity through gases 
we owe a great deal of our knowledge of the nature of 
electrical processes in general, since the conditions here 
are particularly favorable for observation. 


On the conduction of electricity through gases see W. C. D. 
Whetham's "The Recent Development of Physical Science" 
(1904), Chapter V. 

Sir J. J. Thomson's great work "The Conduction of Electricity 
Through Gases" should also be mentioned. 

On conduction in solutions, see Augusto Righi's "Modern The- 
ory of Physical Phenomena (1904), Chapter I, and "The Theory 
of Electrolytic Dissociation," by Harry C. Jones (1900). 



Section 30 

The analogy between the transmission of power by 
electricity and by compressed air really amounts to 
something very close to identity, if the modern view is 
correct. When air is pumped in at one end of a pipe 
the air molecules at that end exert an increased force 
upon the others further along and thus increase the pres- 
sure throughout the system. Similarly, in an electrical 
circuit an increase in voltage may be thought of as cor- 
responding to the introduction of further electrons into 
that part of the circuit which lies just beyond the dynamo 
or battery. These repel neighboring electrons and thus 
the electrical pressure increases all along the circuit. 

Ordinarily the electrons are confined within the body 
of the wire just as the air molecules are held within the 
pipe. Leaks, however, may occur as in the case of 
compressed air systems as shown by the glow which 
sometimes surrounds high tension lines at night. 

The influence of electrons upon each other's motions 
of course depends in large part upon the enormous elec- 
trical repulsions which exist between them. This is not 
so clearly the case with molecules. 


See E. E. Fournier d'Albe, "The Electron Theory" (1906), 
Chapter VH. 

Section 31 

The Principle of the Thermopile. Roughly speaking, 
the number of free electrons in a substance is a measure 
of the lack of affinity of its atoms or molecules for elec- 
trons. The fact that this aftimty varies for different sub- 



[Sec. 31 

stances has some interesting consequences quite apart 
from the production of various degrees of electrical con- 
B ductivity. For example, if two 

metals the atoms of one of 
which have a greater affinity 
for electrons than have those 
of the other, are placed in 
contact the former will appro- 
priate electrons from the 
latter. This is due to the fact 
that the " evaporation of elec- 
trons" from the surface of 
one of the metals is more 
rapid than that from the sur- 
face of the other metal, so 
that the first gives out to the 
second more electrons than 
it receives. Hence the second 
metal becomes negatively 
charged, while the first ac- 
quires a positive charge. 

If we suppose the two met- 
als in question to be in the 
form of horse-shoe shaped 
wires touching each other at 
their extremities it can easily 
be seen that no current of 
electricity will flow through 
the circuit which is thus 
formed, for the reason that 
the electrical forces which ex- 
ist at one junction are exactly 
balanced by those existing at the other junction. Suppose, 
however, that the latter is heated to a temperature higher 


Fig. 21 

This is a symbolic drawing. The 
circle as a whole represents the 
complete electrical circuit, the left 
half being composed of a metal 
which emits electrons freely and 
the right half of one which parts 
with its electrons less easily. If 
the junctions A and E are both at 
the same temperature no current 
will flow, since the tendency to- 
wards a clockwise current which 
exists at E is exactly balanced by 
the opposite tendency existing at 
A. However, when the junction A 
is heated these tendencies are no 
longer exactly in equilibrium and 
electrons move around the circuit 
in the direction of the arrows. It is 
not necessary that the circuit should 
be made up of equal masses of only 
two different metals. It may be 
broken at any point and long wires 
of any sort of conducting substance 
introduced without altering its gen- 
eral principle. 


than that of the former. (See Figure 21.) This will 
bring about a change in the electrical forces at the 
point which is being heated, due in part to the fact that 
the number of electrons thrown off by one metal increases 
with the temperature faster than the number emitted 
by the other metal. The electrical equilibrium of the 
circuit is thus disturbed, and a current will flow, that 
is, electrons will move from one junction towards the 
other. This motion of the electrons carries heat energy 
from the hot to the cold junction so that continued heat- 
ing and cooling is necessary in order that it should per- 
sist. This is the principle of the so-called thermopile. 
The Thermo- Electric Series of the Metals. By studying 
different thermo-electric circuits of the sort described 
above we can arrange a series of metals in which by con- 
tact with a standard metal, each member of the series 
gives a higher voltage than the member preceding it. 
It is found that this series has close affinities with 
another series which is determined by studying the vol- 
tages generated by the same metals in the form of an 
ordinary electrical battery. The exact nature of the 
electro-motive series varies with the temperature, on 
account of the fact that as different metals are heated the 
number of free electrons which they contain in a given 
volume does not necessarily alter in the same way. A 
sample sequence is represented below: 

Metal Relative Potential Difference 

Bismuth' 89 to 97 

Nickel 22 

German-silver 11.75 


Platinum 0.9 

Copper - 1.36 

Zinc - 2.3 

Iron - 17.5 

Antimony - 22.6 to - 26.4 

Tellurium 502. 

Selenium 800. 

Lead is taken as the standard metal. 




On thermo-electricity, consult: E. E. Fournier d'Albe's "The 
Electron Theory" (1906), Chapter V. 

See also: O. W. Richardson's " Aggregates of Electrons," in 
the " Proceedings of the American Philosophical Society" (1911), 
volume 50, pp. 347-366, and 

W. C. D. Whetham's "The Theory of Experimental Electricity" 
(1912), Chapter VI. 

On the evaporation of electrons see N. Campbell's "Modern 
Electrical Theory," second edition (1913), pp. 81-84. 

Section 32 

Electro-positive and Electro-negative Elements.* Chemi- 
cal affinity may perhaps be regarded as another expres- 
sion of the fact that different kinds of atoms have varying 
degrees of attraction for electrons. It has been known 
for a long time that the chemical elements could be 
grouped into three not very sharply defined classes, 
those which were characteristically electro-positive, those 
characteristically electro-negative, and those which might 
be either electro-positive or electro-negative, according to 
the circumstances. Hydrogen and the metals are the 
most powerfully electro-positive of the elements, while 
such substances as oxygen, chlorine, fluorine, etc., are 
strongly electro-negative. As we have seen, the former 
lose electrons easily; the latter, on the other hand, ap- 
propriate with great avidity electrons not their own. 

It is clear that when a neutral atom of hydrogen, for 
example, parts with an electron it must become positively 
charged, and if this electron or another is appropriated 
by an atom of chlorine, for instance, the latter becomes 
negatively charged. Positively and negatively charged 
atoms of this sort will obviously tend to combine, owing 



to the electrical attractions existing between them, and 
these attractions, it seems probable, constitute chemical 

When we study the actual constitution of chemical com- 
pounds we find that the most common compounds are 
made up of just such electro-positive and electro-negative 
components. Common salt, for example, is composed of 
the strongly positive element sodium, combined with the 
strongly negative element chlorine. 

Any Element may be Positive or Negative. However, 
if we make a table of elements which we suppose to be 
electro-positive and another of those which we suppose 
to be electro-negative, we soon discover that, however 
we may construct the table, exceptions always occur to a 
rule which states that only atoms of opposite sign com- 
bine with each other. For a long tune it was thought that 
the fact that two negative or two positive atoms could 
combine chemically was a refutation of the electrical 
theory of chemical affinity, since atoms bearing charges 
of like sign could only repel each other. The electron 
theory removes this difficulty, along with others, for any 
atom can become negatively charged if it can gain an 
electron, or positively charged if it can lose one, and the 
same atom may conceivably suffer either of these changes 
according to the conditions under which it is placed. No 
two of the elements have the same affinity for negative 
electricity, and if any two of them are mixed under proper 
conditions the atoms of the one having the lesser affinity 
will lose electrons while the atoms of the other will gain 
them. Mixed with a different element, the same atoms 
which here gain electrons might lose them, if the element 
in question were the more strongly electro-negative. In 
other words, we can speak of the electro-negativity or 
positivity of the elements in a relative sense only. 



There is one difficulty which remains outstanding, 
however, and that is the explanation of the chemical 
affinity which apparently exists between atoms of the 

same species. We have al- 

-f\ ; ' ready mentioned the fact in 

\ / Section 7 that almost all of 

_ i , the elements form complex 

molecules even in the pure 

state. This implies the exist- 

i !_. ence of chemical affinity be- 

i 1 tween atoms of the same 

/ \ kind. Such affinity is prob- 

' \+ ably to be explained by the 

idea that the positive and 

negative charges within any 

WHICH TWO NEUTRAL AGGRE- atom are not uniformly dis- 

throughout it, and 
OTHER hence that the surface of 

The two arcs represented in the figure an atom is, SO to Speak. 
may be thought of as cross-sections 
through the surfaces of two adjacent mottled. TWO atoms OI the 

same kind may thus stick 

to e ether b y electrical attrac - 

tive components of the other. Taken tion if, as 

as wholes both atoms are electrically , j . _. rtrt 

neutral, but at close range powerful represented Ul Figure 22, 

mutual attractions may still exist be- t hev afe gQ or ; en ted With 

tween their parts. This diagram must mev ' ' ^ Li 

not be regarded as an accurate picture fCSpect to each other that 
of the surface of an atom. 

the negative mottlings of 

one coincide with the positive mottlings of the other, 
and vice versa. 

The "Inert" Elements. Some of the elements, such 
as argon, helium, and the like, are chemically inert, that 
is, they refuse to combine with any other elements. The 
reason for this probably lies in the fact that they are 
incapable of becoming permanently ionized, that is, they 


Sec. 33] SOLUTION 

have neither a very strong tendency to gain additional 
electrons or to part with those which they naturally 
possess, but rather tend to be stable in this respect. We 
have mentioned the fact in Section 6 that the electrical 
character of the elements varies with other of their prop- 
erties according to the law of the periodic table. The 
inert elements all fall into the same family, and each of 
them lies between a strongly electro-negative element on 
one side and a strongly electro-positive element on the 
other, so that it is perhaps not surprising that they should 
be neutral. 


See the reference, already given, to Sir Oliver Lodge's "Elec- 
trons," Chapter XVI, and Norman Campbell's " Modern Electrical 
Theory," second edition (1913), Chapter XIII, esp. pp. 340 ff. 

Section 33 

How Water "Ionizes" Dissolved Substances. It has 
been asserted in the course of our discussion that the 
forces of chemical affinity are probably electrical. If 
this is true, any influence which tends to weaken these 
forces should cause the molecules to fall apart under 
the influence of then* heat vibrations. 

Now all substances possess a property called dielectric 
capacity, which can easily be measured, but concerning 
the exact nature of which we need not here trouble our- 
selves. Suffice it to say that the electrical forces between 
any given set of charged particles become less as the 
dielectric capacity of the medium in which they are 
placed becomes greater. All material bodies have a 
higher dielectric capacity than has empty space and it 
would seem that if two bodies rightly selected could come 



into sufficiently intimate contact with each other the 
large dielectric capacity of the one might bring about a 
separation of the electrical particles making up the other. 
Just this intimacy of contact is assured when one of the 
substances is a liquid and the other is in solution within 
it. Suppose, for example, that a solid like common salt, 
the molecules of which are made up of one atom of chlo- 
rine and one atom of sodium, is dissolved in water. 
According to our view of chemical affinity, these mole- 
cules are held together by the attraction which exists 
between the positively charged sodium atom, Na + and 
the negatively charged chlorine atom, Cl~. If the prop- 
erties of the water cause it to weaken this attraction so 
that the oppositely charged parts of the salt molecules can 
be knocked apart, the solution will contain free charged 
atoms, or ions. This process actually occurs with a great 
many substances which can be dissolved in water. It 
is called technically "electrolytic dissociation." 

The Motion of Ions Through a Solution. When so- 
lutions conduct electricity the current is due to the bodily 
motion of the ions which have been produced by the 
splitting up of the molecules of the dissolved substance. 
We have discussed this matter in Section 29, and have 
also referred to some of the related phenomena in Sec- 
tion 2. 

The electrical dissociation of the molecules of dissolved 
substances, which can easily be proven by experiment 
to exist, furnishes us with another striking verification 
of the idea that chemical affinity depends upon electrical 

The Effect of lonization on Boiling and Freezing Points. 
- When any substance is dissolved in a liquid, the boil- 
ing point of the liquid is raised and its freezing point is 
lowered to an extent which is approximately proportional 



to the number of molecules which have entered into 
solution. The cause of these changes can be stated in 
terms of the kinetic molecular theory. The important 
fact for us to notice here, however, is that when the 
molecules of the dissolved substance are split up into 
ions the number of effective molecules is thereby greatly 
increased, and that if this is the case the effect upon the 
boiling and freezing points should be greater than would 
be expected upon the assumption that no such splitting 
of the molecules occurred. Empirical measurements ap- 
pear to verify this conclusion, and also show the presence 
of an abnormally high osmotic pressure (see Section 19) 
in solutions of electrically dissociated substances. 


Refer to Nernst's "Theoretical Chemistry" (1911), Book H, 
Chapter VII, or Talbot and Blanchard's "The Electrolytic Disso- 
ciation Theory, etc." (1907). 

W. C. D. Whetham's "The Recent Development of Physical 
Science" (1904), Chapter IV. 

A complete book on this subject is Whetham's "Treatise on 
the Theory of Solutions" (1902). 

Section 34 

It is a well-known fact that the chemical elements 
combine with each other in proportions which vary with 
the element which is considered. For example, one atom 
of chlorine normally combines with only one atom of hy- 
drogen, while an atom of oxygen usually combines with 
two of hydrogen. A single carbon atom, on the other 
hand, will as a rule unite with four hydrogen atoms. 
The number of hydrogen atoms with which an atom of a 
given element will combine is called the "valency" of 



the element. Some active elements do not combine 
readily with hydrogen, but these unite with oxygen, the 
valency or combining power of which is known to be two. 
The valency of an element is not perfectly constant, 
but it is nevertheless fairly characteristic. Its magnitude 
probably depends upon the affinity of the atoms of the 
element for electrons. The valency of an element hi any 
specified compound represents the number of electrons 
which one of its atoms has either gained or lost. 


See R. K. Duncan's "The New Knowledge" (1908), 166-167, 
and N. Campbell's "Modern Electrical Theory," second edition 
(1913), pp. 340-350. 

Section 35 

Chemical action is not as straightforward a process as 
would at first appear, since it must depend upon the 
random collision of the reacting molecules or atoms. 

Let us consider carefully a case of chemical combina- 
tion or " synthesis" and see if we can visualize for it 
a reasonable mechanism. 

The Basis of the Law of "Chemical Mass Action." 
We start with a certain number of atoms of the species 
A and an equal number of the species B, which are 
capable of combining with each other in the proportion of 
one to one. However, they cannot do this unless all of 
the individual A atoms come into ultimate contact with 
as many different B atoms. All of the atoms are bounding 
about at random and colliding with each other like the 
members of a panic-stricken crowd, and not every colli- 
sion that occurs is between an A and a B atom. This will 



be especially true after a sufficient number of favorable 
collisions have occurred, so that the reaction has pro- 
gressed some distance, for at this time the mixture will 
contain not only separate A and B atoms but also the AB 
molecules which have been formed. When these collide 
with each other and with the atom, no union occurs, so 
that as the reaction proceeds the number of collisions 
favorable to chemical combination constantly decreases. 

It will be observed that this decrease in the number 
of favorable collisions is caused by a diminution in the 
number of reactable atoms which are present. Now the 
rate or "velocity" of any chemical reaction consists in 
the number of molecules which are formed or decomposed 
in a given time, and this, in turn, must depend upon the 
number of collisions occurring in that time. From this 
it becomes clear that as a reaction proceeds, its velocity 
should constantly diminish, and it can be shown in fact 
that at any time this velocity is proportional to the num- 
ber of active atoms or molecules of each kind which re- 
main. This is the law of chemical mass action, which is of 
the utmost importance in the study of chemical change. 

Chemical "Equilibrium." -The reaction which we 
have considered above would be complete when each of 
the A atoms had combined with a separate B atom. It 
would take a long time for such a reaction to reach com- 
pletion, for the reason that when nearly all of the atoms 
had combined, the remaining ones would be separated by 
a multitude of inert molecules and so would have small 
chance of encountering each other. However, in reality 
most chemical changes are reversible, that is, the mole- 
cules which are formed by the reaction tend to break up 
again and reproduce the original substances. Thus, most 
reactions will consist hi two types of change, a forward 
and a reverse. When the reaction commences, the for- 



ward change is the most rapid because there are more 
molecules to enter into it, but as the reaction progresses 
the products of this change pile up, and consequently 
necessitate a constant increase in the reverse reaction. 
There finally comes a time, of course, when the rates of 
the two opposite changes are equal so that they balance 
each other. At this time the " equilibrium point" of the 
reaction is said to have been reached, and apparently all 
chemical change has ceased. This appearance is deceiv- 
ing, however, for beneath the seeming quiet there goes 
on a ceaseless balanced activity. It is characteristic of 
the modern view of things to suppose that nearly all 
cases of seeming rest are hi reality cases of balanced 


On the law of chemical mass action, see W. Nernst's "Theoret- 
ical Chemistry" (1911), Book II, Chapter I. 

Also: G. Senter's "Outline of Physical Chemistry" (1908), 
Chapter VH. 

Harry C. Jones' "The Elements of Physical Chemistry" (1902), 
Chapter IX. 

Section 36 

The Heat Produced by Chemical Change. It is a well- 
known fact that when chemical changes occur, energy is 
usually liberated in one form or another, most commonly 
as heat. If atoms are to combine, under the influence of 
chemical attraction they must first move towards each 
other hi response to this attraction, and in so doing they 
must acquire energy of motion. Hence, in general, the 
temperature of the reacting substances increases in 
proportion to the strength of the chemical affinities which 

are active. 



The Generation of Light and Electric Current. Chemi- 
cal change may produce electrical effects if the condi- 
tions are arranged as in the ordinary "battery," so that 
charged atoms which are free to move can combine with 
the atoms of a solid electrical conductor and deposit 
their charges, so that the conductor as a whole is electri- 
fied and tends to become the origin of an electrical cur- 
rent. The production of light by chemical change may be 
due to the fact that light is caused directly by the vibra- 
tion of the electrons which are active during the change, 
or it may be indirectly caused through the rise in tem- 
perature brought about by the reaction. 

Conditions Favor ing Chemical Action. Almost all chem- 
ical changes involve not only the formation of new 
molecules but also the breaking up of old ones, and since 
the decomposition of molecules would tend to be favored 
by the collisions of the molecules in their heat motion, 
we should expect for this reason alone that the rapidity 
of a chemical change would increase with the tempera- 
ture. As mentioned in Part I, the transfer of electrons 
from one atom to another must also take place more 
readily at high than at low temperatures, and this, too, 
aids the reaction. 

In the case of reaction between gases, high pressures 
are favorable to chemical reaction, since the more mole- 
cules there are in a given volume the more frequently 
they must collide and hence the greater their chances of 
decomposition and recombination. Another important 
condition favoring chemical change is the presence of 
a catalyzer, which is merely some foreign substance 
capable of accelerating a reaction without being changed 
itself. The exact general mechanism by which catalysis 
is produced is not clearly understood, but it is probable 
that the catalyzer is an ionizing agent of some sort. 



"Chemical energy," which is utilized when coal, for ex- 
ample, is burned under the boiler of a steam engine, is 
really the energy of attraction of charged atoms or groups 
of atoms. Human life and industry, to-day, depend abso- 
lutely upon the presence and application of this energy. 


On the energy relationship of chemical change, consult W. 
Nernst's "Theoretical Chemistry" (1911), Books in and IV. 

Section 37 

The Present Status of the "ALiher" Experiment shows 
that light has many of the properties of some sort of 
wave motion. Since it is difficult to imagine a wave 
without thinking of some substance hi which it is a wave, 
physicists have been accustomed up to recent years to 
assume the existence of an all-pervading aether, the 
undulations of which constitute light and other similar 
disturbances. Of late, however, certain very important 
experimental and theoretical results clustering around 
what is called the " principle of relativity" have thrown 
a great deal of doubt upon the existence of the aether, 
so that it is now advisable to conceive of a light wave in a 
somewhat different way. 

The Nature of Electrical Force Lines. Nearly every- 
one is familiar with the fact that the state of affairs in 
the space around a body which is charged with elec- 
tricity can be represented by what are called "lines of 
electrical force." These lines show in a symbolic fashion 
how another charged body placed in the space in question 
would tend to move, or, in other words, what forces would 
act upon it. It is probable that these lines of force have 



some counterpart in reality, and recent developments- 
make it not improbable that electrons and other charged 
particles of atomic or sub-atomic size are actually centers 
of radial " tubes of electrical force." 

"Kinfe" in Electrical Force Lines. It can be shown 
that lines of electrical force, if they exist, must possess 
" inertia," that is, that they must offer resistance to 
changes in their state of motion or rest. In other words 
they act as regards motion very much like a stiff rope or 
wire attached to the electron or other charged particle. 
Hence if the electron is suddenly set into motion or if, 
when in motion, it is suddenly brought to rest, its lines 
of force will not accommodate themselves to this change 
in motion immediately, but will tend to remain at rest or 
to keep on moving as they did before anything had hap- 
pened to the electron. This means that every time an 
electron is slowed down or is speeded up, and every time 
the direction of its motion is altered, k in k* or curves will 
be formed in its lines of electrical force. The principle 
of the formation of these kinks is essentially the same as 
that of the production of waves in a rope, one end of 
which is shaken. Just as the waves in the rope move 
away from their source at a definite speed, so the kinks 
in the electrical force lines travel away from the electron 
or other charges, with the speed of light. The formation 
of such "kinks" is illustrated in Figure 23. 

If the charge which is under consideration vibrates 
continuously back and forth, a series of kinks will obvi- 
ously be formed in its force-lines, and these will con- 
stitute waves. As a matter of fact light and many other 
forms of radiation are probably made up of a series of 
just such kinks or curves. 

Electrons probably do not vibrate or oscillate as much 
as we were once inclined to believe. It seems more 



[Sec. 37 

likely, in the light of recent developments, that they 
move in "jerks," dropping suddenly from one position 
to another without an ensuing reverse motion. If this 

Fig. 23 


The diagram at the left represents a charge of electricity with its radiat- 
ing lines of forces. We will suppose this charge with its lines to be mov- 
ing uniformly in the direction indicated by the arrow in the diagram at the 
right. When the charge is suddenly brought to rest the "lines" have a 
tendency to continue in motion, and do so until, so to speak, the news of 
the stopping of the charge has reached them. This "news" travels out- 
wards from the charge with the velocity of light, along with the "kinks " in 
the force-lines which result from the discrepancy between the actual and 
the "expected" position of the charge. These kinks contain electro- 
magnetic energy and constitute light and other forms of electro-magnetic 
radiation. Such radiation is produced whenever any change whatsoever 
occurs in the state of uniform motion of an electrical charge. 

is true it means that the ordinary laws of motion do 
not apply without modification to the motion of electrons. 
This question is further discussed in Section 54. 


On the electrical theory of light, consult Righi's "Modern The- 
ory of Physical Phenomena" (1904), Chapter II. 

The theory of "kinked" force-lines above discussed was first 



developed by J. J. Thomson and one of his simplest accounts of 
it will be found in his "Electricity and Matter" (1904), Chapters 
I-in inclusive. 

See also W. C. D. Whetham's "The Theory of Experimental 
Electricity" (1912), Chapter IX. 

Section 38 

The rapidity with which any vibrating body moves 
back and forth in its path depends upon the magnitude of 
the forces which are acting upon it. Magnetic forces are 
known to act upon moving electrically charged bodies and 
hence we should expect that if there are electrons vibrat- 
ing or moving in any way within sources of light, the 
application of a magnet to such a light source would alter 
the rate of vibration of these electrons. If this occurs, 
the vibration time, and hence the " wave-length" of the 
light, must also be changed. From the mathematical 
theory of electricity it is possible to calculate the exact 
change which should occur, assuming a vibrating particle 
of known charge and mass. Conversely, if we know the 
change hi the character of the light we can estimate the 
charge and mass of the electrical particle which is emit- 
ting the light. 

A very large number of observations have been made 
on the effect produced by magnetic forces upon the light 
given off by many different substances in the glowing 
state. All of these observations show that, certainly in 
the majority of cases, the vibrating, or otherwise moving, 
particle is an electron. 

The alteration of wave-length of the light emitted by 
a glowing body under the influence of magnetic forces is 
known to physicists as the "Zeeman effect," after its 
discoverer Paul Zeeman. (See Figure 24.) 



[Sec. 39 

It has been shown by J. Stark that the mode of vibra- 
tion of the electrons within an atom can also be modified 
by the application of a strong electrical field. 

A B 

Fig. 24 

The two lines A and B in 1 are svpposed to be two lines in the spectrum 
of a luminous element, such as hydrogen or mercury vapor. When a 
powerful magnet is applied to the glowing element these spectral lines 
break up into "triplets" or complex groups of triplets, as shown in 2. 
This is called the Zeeman effect, and in its simplest form is readily ex- 
plained by the electron theory. 


See Sir Oliver Lodge's " Electrons" (1907), Chapter XI; E. C. C. 
Baly's " Spectroscopy " (1905), Chapter XIV, and Norman Camp- 
bell's "Modern Electrical Theory," second edition (1913), pp. 

Zeeman's own account will be found in his "Researches in 
Magneto-optics" (1913). 

Section 39 

Temperature Radiation; . The Spectral "Distribution 
Curve." -There are various special conditions under 
which material bodies emit light. The one which is most 
familiar is that of high temperature. 

We have already stated that the electrons in bodies 
take part in the heat vibrations along with the atoms and 



molecules, and if this is true they must constantly give 
off electrical waves. The length of these waves must 
obviously depend upon the rapidity of the vibrations, and 
this, in turn, increases with the temperature. Hence, 
as the temperature of a body containing electrons is 
raised, the preponderating length of the waves which 
it gives out should become less. This is exactly what 
occurs in nature. 

When a body is being heated the first perceptible radia- 
tions which it emits are "heat waves." When the body 
becomes "red hot" it is giving off the longest of the light 
waves, and "white heat," which every one recognizes 
to be hotter than "red heat," is only possible when green 
and blue light have been added to the red. The wave- 
lengths of the blue and green are much shorter than that 
of red light. With still further increases in temperature, 
the radiation begins to include still shorter waves which 
make up the so-called "ultra-violet" light. 

On account of the relationship which holds between 
the temperature of a hot substance and the color of the 
light which it emits it is possible to get an idea of this tem- 
perature by means of observations upon the color of the 
body. The so-called "optical pyrometer" is an instru- 
ment based upon this principle, which does for very hot 
bodies what an ordinary thermometer does for cooler 

As we have seen in Section 23, all of the molecules, and 
hence all of the electrons, in a body are not moving at 
the same velocity even if the temperature is throughout 
what we call "uniform." Some are moving faster and 
others slower, according to the "law of distribution of 
molecular speeds," as explained in the Section referred 
to. Owing to this fact we should not expect the light from 
a body at a given temperature to be all of the same wave- 



length. Supposing that the particles whose vibrations 
are responsible for the light are electrons, it is possible 
to calculate the wave-length of the light corresponding to 
the " average molecular energy" which is characteris- 
tic of a given temperature. Since there are more elec- 
trons moving at a velocity corresponding to approximately 
this energy than at any other (approximate) velocity, we 
should expect most of the light to be of (approximately) 
the calculated wave-length. Measurements verify this 
expectation and also the idea that the electron is the ac- 
tual source of the radiation. The form of this " curve 
of distribution" of energy in the spectrum of a solid at 
various temperatures is shown in Figure 25. 

But there are electrons moving at speeds both higher 
and lower than this "average speed," and hence there 
should be light to correspond, although such light should 
be less intense, the further its wave-length departs from 
that belonging to the "average speed." Observation 
validates this conclusion also. 

However, the so-called law of the distribution of light 
intensities along the spectrum for any given tempera- 
ture is not exactly what should be expected from the 
molecular theory, in its ordinary form, and the efforts of 
physicists to explain its deviation from the expected form 
have finally culminated in the modern "quantum" 
theory of light which is discussed in another place (see 
Section 54). 

The theory of temperature radiation is based pri- 
marily upon the conception of a "black body" which is 
a body absorbing all radiation impinging upon it and 
showing a minimum of selectivity in its emission of 

The Emission of Light by Gases. Gases as well as 
solids give off light under the right conditions. It is only 


Sec. 39] 



Fig. 25 


The numbers along the horizontal scale represent the wave-length of 
the radiation (light or heat) in thousandths of a millimeter. The vertical 
scale represents the relative intensity of the radiation. It will be observed 
that the curves for the higher temperatures (given in degrees Centigrade) 
have their maxima at points corresponding with shorter waves than those 
characteristic of the lower temperatures. The meaning of this fact is 
explained in the text. These curves are those of the so-called "black 
body radiation." 



when a substance is in the gaseous state that its charac- 
teristic "spectrum" can be obtained distinctly. 

It has not generally been considered possible to cause 
a gas to glow simply by heating it. The reason for this 
is to be found in the fact that under ordinary conditions 
gases contain very few free electrons or ions, and that even 
high temperatures will not produce a sufficient number 
of these to make the gas luminescent. 

In order to accomplish this end, it is necessary either 
to send an electrical current through the gas or to permit 
chemical action to take place within it. We have seen 
in Section 27 that both of these conditions favor ioniza- 
tion. Ions can be formed in a gas only when electrons are 
" knocked out" of the atoms which make up the gas, and 
it is the change in the "mode of motion" of the electrons 
which occurs either when they are being ejected from or 
when they return to the atoms which gives rise to the glow 
that accompanies (say) the electrical discharge through 
the so-called " vacuum tube." The light of the familiar 
" mercury vapor arc" depends upon the same principle. 

"Line Spectra" - The light which is given off by 
glowing gases differs from that emitted by solids hi 
respect to its " distribution " along the spectrum. Prac- 
tically all of the energy in the former case is concentrated 
about certain definite wave-lengths, the positions of which 
are not only constant for a given gas, but are different 
for different gases. The "spectrum" of a glowing gas, 
then, shows a series of "lines" of colored light in place 
of the continuous rainbow band which is produced when 
light from a white hot metal is passed through a prism. 
This special nature of the spectra of substances in the 
gaseous condition must be attributed to the fact that the 
electrons in such substances have much less freedom of 
movement than have those within a metal, so that they 



can only execute vibrations of a few definite frequencies, 
as determined by the structure of the atoms and mole- 
cules of the substance. 

The study of the line spectra of the various elements 
hi gaseous form has shown that in any single element 
the lines can be arranged into "series," such that the 
positions of the individual lines hi these series that 
is, the wave-lengths of the lights composing them can 
be calculated from relatively simple mathematical for- 
mulae. The exact nature of these formulae differs from 
element to element, although it retains important points 
of identity throughout. It has for some time been recog- 
nized, on the basis of the Zeeman effect (which we have 
discussed in Section 38), that the partial cause of this 
identity lies in the fact that, in all of the cases consid- 
ered, the light is emitted by electrons. Only recently, 
however (see Section 53), have physicists been able to 
suggest a way in which the relatively simple electronic 
structure attributed to such atoms as that of hydrogen 
could be consistent with the very complex nature of their 
line spectra. 


A complete but elementary discussion of the various forms of 
electro-magnetic radiation is given in S. P. Thompson's "Radia- 
tion" (1898). 

A more modern treatment is the excellent one by N. Campbell 
in his "Modern Electrical Theory," second edition (1913), Chap- 
ters IX and X. 

Section 40 

When a beam of white light, such as ordinary sunlight, 
is passed through a glass prism it is broken up into the 
so-called prismatic colors, which are arranged in the order 



of their wave-length. The shortest waves are repre- 
sented by the violet light, and the longest by the red. 
The wave-length of the latter is about twice that of the 

The arrangement of the colors in the spectrum produced 
by a prism is not such that the distance of each color from 
the end of the spectrum is proportional to its wave-length. 
This relationship does hold, however, in what is called a 
"normal" spectrum. If a spectrum of this latter sort 
about a yard long could be extended so as to include all 
known electro-magnetic radiations (among them the 
Hertzian waves) it would become over five million miles 
in length. 

Of those waves which are shorter than light, the most 
familiar are the so-called "ultra-violet" waves, which 
lie just beyond the violet end of the spectrum, and the 
" X rays," which recent experiments indicate to be differ- 
ent from ordinary light chiefly in the possession of an 
extremely short wave, or "kink" (see Section 37), about 
one thousandth that of ultra-violet light. With the 
"X rays" we have to include the so-called "gamma 
rays" from radium (see Part I). The existence of these 
rays can be detected by their chemical effects, for example 
by their power to produce pictures or shadows on a photo- 
graphic plate. 

The waves which are longer than light comprise the 
so-called "heat rays," which affect our temperature 
sense, and the Hertz waves," which are employed in 
wireless telegraphy. 

All of these waves travel at the same speed hi empty 
space, D/Z., at the rate of 186,300 miles per second. The 
length of the shortest visible light wave is about one 
hundred-thousandth of an inch. The longest Hertzian 
waves measure over a mile from crest to crest. On ac- 



count of the tremendous speed at which light travels 
the highest speed known to science, and perhaps the 
highest possible speed the rapidity of vibration, or 
the " frequency" of light as it passes through a fixed 
point, is extremely great. About eight hundred trillion 
waves of violet light would pass through such a point 
in a second. The extreme brevity of the interval of time 
required for the passage of a single wave of this sort 
or for the completion of a single oscillation of the generat- 
ing electron may perhaps be realized better when it is 
said that one eight-hundred-trillionth of a second is a 
vastly smaller part of a second than a second is of the 
whole of historic time (i.e., one two-hundred-and-fifty- 


A very popular account of the properties of light is given in 
" Light, Visible and Invisible" (1910), by Silvanus P. Thompson. 

Section 41 


The "Selective Absorption" of Light. When light 
passes through a semi-transparent body it always be- 
comes less intense, on account of the absorption which 
takes place. If the original light is colorless we generally 
find that it is more or less colored when it comes out 
of the body. This is due to the fact that white light 
is a mixture of lights of many different wave-lengths, 
and that these lights are not all absorbed hi equal 

The reason for this inequality of absorption is to be 
found in the general explanation of the absorption of 


COLOR [Sec. 41 

light which is given in Part I. A body absorbs only such 
light as can produce response in its electrons. If the 
internal forces (see Section 8) of the molecules of a given 
body permit the electrons which they contain to respond, 
to an appreciable extent, only to red light, a beam of 
white light passing through such a body will be colored 
blue-green, because the red light is removed by absorp- 
tion and its complementary blue-green is thus left 
unbalanced. Generally a body will strongly absorb light 
of several quite different wave-lengths. 

The Sensations of Color. The basis of the sensations 
of color is not to be looked for in the nature of light, so 
much as in the nature of the effects which light produces 
in the retina of the eye and in the nerves which are con- 
nected thereto. Physics as such offers no explanation of 
the fact that light of one wave-length gives us a sensa- 
tion quality almost wholly different from that produced by 
light of another wave-length. Neither does it account for 
the fact that a mixture of lights of many different wave- 
lengths gives white. These are problems in physiology 
although their solution is, of course, closely connected 
with the physics of light. 

How Color is Produced by Reflection. When light is 
colored by reflection from the surface of a body, as for 
example, from a piece of green paper, we must suppose 
the process to be in reality an absorption phenomenon, 
since most of the light undoubtedly penetrates the body 
to a certain extent before it is reflected. It thus passes 
through a portion of the body in two directions, and dur- 
ing this passage is subjected to the ordinary conditions of 

Bodies differ in their power to reflect light, primarily 
on account of the fact that the electrons which they 
contain are not similarly conditioned by their surround- 

[ 158 ] 


ings. The best reflectors will in general be the metals, 
since these contain free electrons in large numbers. 


For a semi-popular discussion of color and absorption, refer 
to "Light" by R. C. Maclaurin, Chapters II and in. See also 
Franklin and MacNutt's "Light and Sound" (1909), especially 
Chapter X. 

A very recent and comprehensive exposition of color problems 
is that of M. Luckiesh, "Color and Its Applications (1915). 

Section 42 

How Columns of Light are Bent. Light may be 
thought of as moving in columns or "pencils" the fronts 
of which are perpendicular to the direction of motion of 
the light. When the front of a column strikes the surface 
of a transparent body the edge which meets the surface 
first must be retarded in its motion if, as is generally the 
case, light moves slower in the body than it does in empty 
space. Hence, the edge in question falls behind the other 
parts of the front and the plane of the front is rotated. 
Since in general the column will move in a direction at 
right angles to the plane of the front, the light is bent at 
the surface of the body. If the column of light strikes the 
surface at right angles there will be no bending because 
all parts of the front will hit the surface at the same time. 
The more acute the angle of impact, the greater will be 
the bending. 

This bending of a light column or pencil when it passes 
through a surface obliquely is called refraction. As is 
well known, refraction lies at the basis of the effects pro- 



duced by all lenses and prisms. To explain the details 
of the process is beyond the scope of this book, since 
they are complicated and do not depend in any especially 
significant way upon the modern theory of matter, but it 
should be noted that refraction depends solely on the 
fact that the velocity of light is different inside and out- 
side of the medium in question. 

"Dispersion." -The same substance refracts light 
waves of different lengths to different extents. This 
produces what is known as dispersion, a process upon 
which the formation of spectra by prisms is based. It 
has been shown experimentally that the degree in which 
a given substance refracts light of any specified vibration 
period is closely related with the natural period of vibra- 
tion of the molecules of the refracting substance itself. 
As the vibration period of the transmitted light approaches 
that of the substance, the refraction increases enormously, 
and then changes suddenly. It is this difference in the 
refractive power of a single substance for different wave- 
lengths which makes possible the " dispersion" of color, 
as in the ordinary prismatic spectrum. 

"Dielectric Capacity" and the "Index of Refraction" 
The degree to which a substance refracts light of speci- 
fied wave-length is called its index of refraction for this 
light, and it has been shown that a definite relation- 
ship exists between the dielectric capacity of a substance 
and its index of refraction. (See Section 33 for another 
connection of dielectric capacity.) For many solids and 
liquids this relationship is a complicated one when the 
frequency of the light considered is close to that of the 
molecules of the substance itself. However, in the case 
of gases, and if sufficiently long waves are used, for all 
bodies, the refractive index is found to be proportional to 
the square-root of the dielectric capacity. 



The dielectric capacity of a body measures the ease 
with which the electrical components of its molecules 
undergo temporary separation (without decomposition of 
the molecule) through the activity of outside electrical 
forces. In general, the more the particles of a substance 
respond to the force of a light wave the greater is the 
effect upon the speed of the wave. Hence it is easy to 
see why, within the above-mentioned limits, a substance 
which has a high dielectric capacity should also have in 
general a high index of refraction. 

Both refraction and absorption occur to the greatest 
extent when the frequency of the light which is passing 
through a body most closely approximates the natural 
frequency of vibration of the molecules of the substance, 
since under these conditions the response of these mole- 
cules is the greatest possible. 


On refraction and reflection, consult R. C. Maclaurin's "Light," 
Chapter VI. 

Section 43 

The fact that a magnetic field is produced by the mo- 
tion of an electrically charged body was proven by the 
late Professor Rowland of Johns Hopkins University. 

Professor Rowland's experiment was a very simple 
one. Everybody is familiar with the fact that the presence 
of a magnetic force can always be detected by the deflec- 
tion which it causes in the position of a compass needle. 
Some strips of gold-leaf were cemented upon a hard 
rubber disk, and after they had been charged with elec- 
tricity the disk was rotated very rapidly. The needle of 



a delicate compass placed near by showed a distinct 
deflection, which could only be ascribed to the magnetic 
forces generated by the motion of the charged strips of 

The relation between the direction of the magnetic 
forces and that of the moving electricity or electric current 
is shown in Figure 26. 

Fig. 26 


Magnetic forces exist around every moving electrical charge. If the 
charge is positive and is moving in the direction of the large arrow the 
magnetic forces will possess the arrangement and direction indicated by 
the black circle and its small arrows. If the charge is negative it must 
move in the opposite direction to produce the same magnetic effect. 


On the relation between electricity and magnetism, see Oliver 
Lodge's "Modem Views of Electricity," Chapter VII. 

Section 44 


The principle which is employed in the dynamo for 
the generation of the electrical current is illustrated in 
certain modern experiments and observations to which 
we have already referred in several places. 



In Section 25 we have mentioned the fact that the 
so-called cathode rays, which in reality consist of very 
rapidly moving electrons, are capable of being deflected 
by a magnet. Where the rays impinge upon the walls of 
the "vacuum tube" in which they are produced they 
cause a bright spot of light. If a strong magnet is brought 
near the tube this spot of light is seen to move. 

This deflection of the rapidly moving charges in the 
cathode rays has been very carefully studied and the 
results embodied in what is believed to be one of the most 
fundamental and exact of electromagnetic laws; "The 
Law of the Deflection of Moving Charges in a Magnetic 


See J. J. Thomson's "The Discharge of Electricity Through 
Gases" (1898), Chapter on the "Cathode Rays," p. 137; and 
Oliver Lodge's "Electrons" (1906), Chapters VTH-XIX inclusive. 

Section 45 

Dia- and Para-magnetism. One ordinarily thinks of 
the vast majority of substances as "non-magnetic." 
Iron and steel seem to be marked out from other bodies 
by their possession of magnetic powers. As a matter of 
fact, however, every known substance possesses magnetic 

There are two kinds of magnetism, dia- and para- 
magnetism. When a dia-magnetic body is placed in the 
field of a strong magnet its long axis tends to turn so as 
to be at right-angles to the direction of the lines of mag- 
netic force. Para-magnetic bodies, on the other hand, 
tend to place their long axes parallel to their lines of 



The fact that all bodies have magnetic properties in- 
dicates from the present point of view that they all contain 
electrons in motion. Whether a particular body is dia- or 
para-magnetic probably depends upon the exact way in 
which the electrons are moving and upon the conditions 
which limit this motion. 

Permanent Magnetism. Iron, cobalt and nickel differ 
from other substances hi their power to acquire strong 
permanent magnetism. This property may be explained 
in the following manner. If there are electrons rotating 
within or about the atoms of a substance, each atom will 
behave like a very small magnet. But in a visible body 
there would be so many of these minute magnets with 
their positive and negative poles pointing "every which 
way" that the resulting outside effect would be prac- 
tically nil. 

In order that such an external effect should be pro- 
duced it would be necessary for a number of the little 
magnets to point hi the same general direction, so that the 
individual influences would be added to, instead of neu- 
tralizing, each other. To bring the atoms into line in 
this way an outside magnetic force might be applied, 
for it is known from experiments with a compass needle 
that one magnet can cause another to turn upon its axis. 
It is probably some such reaction of the atomic magnets 
as this which produces all of the phenomena of dia- and 
para-magnetism. In the majority of bodies the atoms do 
not remain in magnetic line when the outside force is 
removed. But in others they do tend to remain in 
line and these latter are said to possess permanent 

If magnetization actually does involve a turning of the 
molecules on then* axes, we should anticipate that a cer- 
tain amount of the energy which is used in the process 



might be converted into heat, since additional motion is 
imparted to the molecules. This effect can actually be 
observed in the case of the more magnetic substances, 
and is at the basis of a troublesome loss of energy in 
dynamo work technically called " hysteresis." 


An exhaustive discussion of "Magnetism" by Shelf ord Bidwell 
will be found in the eleventh edition of the " Encyclopaedia Bri- 
tannica." For a simple and briefer account see Norman Campbell, 
loc. cit., Chapter V. 

Section 46 

The Discovery of the Radio- Elements. The first sub- 
stance found to be radio-active was uranium. The dis- 
covery was made by Becquerel in 1896. M. and Mme. 
Curie, however, soon found that the ore from which 
uranium was extracted was four times as radio-active as 
pure uranium itself, and this observation led to the dis- 
covery of radium, a new chemical element having a 
radio-activity over a million times greater than that of 
uranium. Later on the same investigators laid bare fur- 
ther and even more powerful radio-active bodies: polo- 
nium and actinium. Another element, thorium, already 
known to chemists, was found by Schmidt to be radio- 
active to about the same extent as uranium. 

"Disintegration Series" A complete list of the radio- 
active elements is given in Table II of Section 5. Most 
of the substances named in this table are what may be 
called " radio-active products," that is, they are bodies 
which are produced in the course of radio-activity. Ra- 
dium, for example, apparently breaks up into helium and 
" niton," both of which are gases. The emanation 



(niton) decomposes, in turn, forming a solid substance, 
radium A, which is soon converted by a similar change 
into radium B, and so on. It is supposed that the final 
product of this series of changes is chemically identical 
with lead. 

Similar " disintegration series" of substances are de- 
rived from uranium, thorium, and actinium, as shown in 
the table (II, Section 5). A series may " branch" at 
certain points. It is now believed that both radium and 
actinium are ultimately derived from uranium, by such 
a process. 

Besides these special radio-active bodies others have 
now been shown to possess slight radio-active powers. 
Many investigators believe that all substances are some- 
what radio-active, but it is not yet entirely certain whether 
the faint activity which exists, is an intrinsic property of 
the substances or whether it is to be ascribed to the 
presence within them of small traces of the radio-active 
bodies proper. 

The Law of Decay of Radio-Adice Substances. As 
stated in Part I, the rate of decay of a radio-element, 
so far as yet found, is independent of all external condi- 
tions. The law of this decay is that, for a given element, 
the same fraction of any specific volume will always break 
up in the same period of time. Thus, if we should start 
with an ounce of radium A, it would be hah* gone at the 
end of three minutes. In three minutes more one-half 
of the remainder or one-quarter of the original amount, 
in addition would have broken up, and so on. It is 
clear that a law of change of this sort would theoretically 
never lead to the total disintegration of any given quantity 
of the substance, however small. This is why the "life " 
of a radio-element is stated in terms of the time required 
for one-half of a given amount of it to decay. 



These " half-times" vary from about twenty-six bil- 
lion years in the case of thorium to only one ten-billionth 
of a second in the case of radium C'. The intensity of 
the radiation from any radio-active substance is naturally 
inversely proportional to the time required for a quantity 
of it to decompose. It has been shown by Geiger and 
Nuttall that the shorter the period of lif e of a radio-element 
the faster its alpha particles move. There is a similar 
relationship for the beta particles. 

The Position of the Radio-Elements in the Periodic Table. 
The position of the principal radio-active elements in 
the periodic table is especially worthy of notice. It will 
be seen (Section 6) that not only are they among the 
heaviest of the elements, but that all of the heaviest ele- 
ments are radio-active. Apparently the relative insta- 
bility of the radio-elements is a corollary of complexity of 
internal structure, and is analogous to the unstable char- 
acter of the higher chemical complexes, such as the 
organic compounds of which living bodies are made up. 
However, mere weight is not the only factor involved, 
since uranium, the heaviest element known, and the 
parent of the majority of the radio-active substances, is 
one of the least active of them all. It is estimated that 
the time of decay of uranium is long compared with the 
age of many of the minerals hi which it is found. If it 
were not for this fact, and the similarly low activity of 
thorium, all signs of radio-activity would have disap- 
peared from the earth long ago. It is probable that other 
series of radio-elements, perhaps of even greater atomic 
weight, have existed in the past, but have left no recog- 
nizable traces behind them. 


E. Rutherford: " Radio- Active Substances and their Radiations" 
(1913), (Standard treatise). 



F. Soddy: "The Interpretation of Radium" (1912) (popular 
lectures), and "The Chemistry of the Radio-Elements" (1915). 

R. K. Duncan: "The New Knowledge" (1905), Parts 4 and 5, 
pp. 87-193 (popular). 

C. W. Raffety: "An Introduction to the Science of Radio- 
Activity" (1909). 

A. T. Cameron: "Radio-Chemistry," (1910). 

Section 47 

We have seen in Part I and in Section 25 that moving 
electrically charged bodies can be turned from their 
straight-line paths, or deflected, by the action of a mag- 
net. Hence it is possible to get a first indication as to 
which, if any, of the rays from radium are electrical 
particles, and which are electrical waves, by placing a 
magnet across their paths. The first type of rays will be 
deflected while the second will not. The extent to which 
they are deflected, when combined with certain other 
measurements which have been mentioned hi Section 25, 
shows the speed at which they are travelling, as well as 
the sign of their charge and the ratio of this charge to 
their mass. 

When an experiment of this sort is tried upon a bundle 
of rays from radium, certain of the rays are found to be 
uninfluenced and to move on in a straight line as before. 
Another set is deflected slightly, while a third suffers 
very marked change in path. The last two are diverted 
in opposite directions, which proves then- charges to be 
positive and negative respectively. The first type of 
rays are the " gamma rays," the second the " alpha 
rays," and the third the "beta rays." The methods 
which are used in measuring the speeds, charges and 



masses of the alpha and beta particles are similar to those 
described in Section 25 for the " cathode rays." 

The fact that the beta particles can penetrate even thick 
pieces of solid matter is proven by simply directing a 
pencil of beta rays towards a plate composed of the solid 
substance in question, and noticing that the rays appear 
in a somewhat diffused and attenuated state on the other 
side of the plate. 

As already mentioned, C. T. R. Wilson has devised a 
method by means of which it is possible to photograph 
the paths of single alpha particles in their motion through 
a gas. 


The methods employed in the study of radio-activity are popu- 
larly discussed by R. K. Duncan in his "New Knowledge" (1908), 
Part IV. 

Section 48 


The fact that the alpha rays are atoms of helium was 
proved in a very striking and simple experiment by Ernest 
Rutherford. Helium is a gas, and it is a well-known fact 
that every gas when subjected to the action of an electri- 
cal discharge gives off light of a peculiar color, which can 
be split up by a glass prism into the so-called " spectrum" 
of the gaseous substance (see Section 39). Rutherford 
very carefully removed all traces of helium from a large 
glass tube and then placed within this tube a smaller one 
of very thin walls containing gaseous helium. After per- 
mitting the apparatus to stand for several days he passed 
an electrical discharge through the larger tube, and ex- 
amined the light to see if the spectrum of helium was 
present. He found it to be absent, showing that ordinary 



helium could not pass through the walls of the smaller 

He now replaced this latter tube by an identical one 
containing not helium but radium emanation, which is 
constantly throwing off alpha rays. Since the walls of the 
tube were thin the great speed and energy of the particles 
in the rays enabled them to penetrate the walls and enter 
the atmosphere of the larger vessel. 

After the apparatus had stood under these conditions 
for a period of time Rutherford again passed an electrical 
discharge through it, and found that it distinctly showed 
the spectrum of helium. This seems to be conclusive 
proof that the alpha ray particles are actually atoms 
of helium. 

The mass or weight of the alpha particles has also been 
measured and has been shown to be of a magnitude in 
harmony with this conclusion. 


See Rutherford's " Radio-Active Substances and their Radia- 
tion" (1913), pp. 137-140 and Chapter XVII. 

Section 49 

We have seen in Section 37 that electrical waves can be 
regarded as kinks hi lines of electrical force which are 
produced whenever the velocity of motion of an electri- 
cally charged particle is altered. Now we know that the 
beta particles bear electrical charges, and that they are 
suddenly emitted from the atoms of radio-active sub- 
stances at a tremendous speed. In accordance with the 
theory, then, they ought to give rise to electrical waves 
of very high frequency, that is, the kinks which are pro- 



duced in their lines of force should be exceedingly sharp. 
This conclusion seems to harmonize with what we know 
about the gamma rays, which are apparently made up of 
the electrical waves in question. 

When the beta particles are stopped in their headlong 
flight by striking the atoms of some other substance we 
should expect further gamma (or X) rays to be produced, 
because any change hi the velocity of one of these par- 
ticles should give rise to an electrical wave, whether the 
change be of the nature of an acceleration or a retarda- 
tion. It has been found by experiment that such rays 
are actually produced under the circumstances specified. 
They belong to the class of " secondary rays" hi which, 
also, must be included the further beta radiation which is 
set up by the gamma rays themselves when they are 
absorbed by material bodies. 

Another type of secondary rays produced under en- 
tirely analogous conditions and probably of the same gen- 
eral nature are the well-known "X rays." Certain very 
recent experiments which prove beyond a doubt that 
the latter, and probably also the gamma rays, are not 
moving material or electrical particles, in the usual sense, 
will be discussed in Section 55. In that place, also, will 
be considered the explanation of the great power of 
gamma and X rays to penetrate bodies opaque to ordi- 
nary light. 


Concerning the gamma rays, see R. K. Duncan's "The New 
Knowledge" (1908), pp. 109-112. Also Norman Campbell's "Mod- 
ern Electrical Theory," second edition (1913), pp. 273 /. 



Section 50 

The enormous energy of the inner constitution of the 
atom is probably very closely connected with the great 
stability of atoms in general. Even the atoms of radium 
are only relatively unstable, since if we consider any 
single radium atom it has what we might call an "ex- 
pectation of lif e " of several thousand years. The great 
stability of the atom is due to the fact that the forces 
which hold the parts of the atom together are very great, 
and the magnitude of these forces is closely associated 
with that of the intra-atomic energy. 

We have several times spoken of the forces of chemical 
affinity and cohesion as residual in character, as repre- 
senting the attractions which are, so to speak, "left over" 
after the parts of the atom have been cemented together. 
If chemical energy (for example) is a residue of the intra- 
atomic energies we should expect it to be relatively much 
smaller than these energies, just as the stability of the 
molecule is relatively much less than that of the atom. 
We have become accustomed to the quantities of energy 
liberated in chemical changes and have taken them as 
standards, so that when we come to consider the primary 
energies of the atoms these seem unbelievably great. 


Concerning intra-atomic energies read R. K. Duncan "The 
New Knowledge," Part 5, Chapter HI. The energy liberated in 
radio-activity is discussed by J. J. Thomson in his "Electricity 
and Matter" (1904), pp. 152 /. 



Section 51 

As nearly everyone knows, the element potassium is 
a constituent of ordinary caustic potash. Yet this common 
element has been shown by Norman Campbell to be 
definitely radio-active. The rays which it emits appear 
to be principally of the "beta" type, the intensity of the 
rays from potassium being about one one-thousandth 
that of the beta rays from uranium. All of the potassium 
salts are radio-active, and thus far no evidence has been 
adduced to show that this activity is due to impurities. 

Other of the so-called alkali metals, for example rubid- 
ium, have been shown to possess slight radio-activity. 


See Norman Campbell: "The Radio-Activity of Potassium" 
in the Proceedings of the Cambridge (Eng.) Philosophical Society 
for 1908, Vol. 14, pp. 657-567. Also Campbell's " Modern Elec- 
trical Theory," second edition (1913), pp. 187-188. 

Section 52 

We have seen in Section 39 that all of the elements 
possess characteristic " spectra" which consist of series of 
lines. For most elements these series are quite complex 
and the positions of the separate lines in the spectrum, 
that is, the wave-lengths of the lights which compose 
them, do not change. However, the exact number of 
lines which are present depends to a certain extent 
upon the conditions under which the element is made 
luminous. The spectrum from a flame has fewer lines 
than that from an electric arc. 

Now astronomic observations have shown that in the 


light from many stars the spectra of certain elements 
are curiously incomplete. In general, the hotter a star 
is the more incomplete the spectra of its elements appear. 
If we suppose that the different sets of lines hi the spec- 
trum of the element are produced by the vibrations of 
different electrons, or systems of electrons, within the 
atom, it is natural to infer that the simplification of the 
spectra in the hottest stars stands for an actual breaking 
up of the atoms in these stars. 

But in addition to this it has been shown, by Sir Nor- 
man Lockyer on the basis of spectroscopic evidence, that 
hi the very hottest stars are to be found only the simplest 
elements, such as hydrogen, helium, etc., along with 
partly formed elements of higher atomic weight. The 
cooler a star is the more elements it contains, and the 
higher are the atomic weights of these elements. 

These facts suggest that the elements are undergoing 
an actual evolution in certain of the heavenly bodies, an 
evolution which depends primarily upon the fact that 
these bodies are passing through a process of cooling. 
The lightest and simplest elements are formed first, 
and after them the heavier and more complex ones. 
Among the latter are the radio-active substances. 


Sir Norman Lockyer's own account will be found in his "Inor- 
ganic Evolution, as Studied by Spectrum Analysis." A simpler 
presentation of the facts is given by R. K. Duncan in Part VI of 
"The New Knowledge" (1908). 

Section 53 

Thomsons Theory. Up to very recent times the most 
promising conception of atomic structure was that elabo- 
rated by Sir J. J. Thomson in a highly mathematical 



paper published in the English Philosophical Magazine, in 
1904. Although it is practically certain that the theory, 
as originally stated, is not accurately true, it must never- 
theless be admitted that no other view has appeared 
which gives us an equal feeling of insight into the mystery 
of the chemical elements. 

On the basis of the known laws of electrical attraction, 
Thomson calculated the constitution of the series of hy- 
pothetical atoms which would be generated by the suc- 
cessive addition of electrons to a large sphere of positive 
electricity always of sufficient charge to just neutralize 
the electrons. For simplicity, he assumed the electrons 
to be concentrated in a single plane. He was able to 
show that the electrons would arrange themselves into 
rings and that with an increase in the total number there 
would be a periodically recurring tendency for fresh rings 
to be formed, in addition to those already present. How- 
ever, the latter, also, would be obliged, from time to time, 
to increase their electronic contents in order to maintain 
the stability of the system. 

With the formation and development of each new ring 
the properties of the atoms might be expected to repeat 
to a limited extent, those which were passed through in 
the development of the previously formed ring. The 
presence of the latter owing to the general modifica- 
tion of the forces of the system which it would involve 
-would preclude a*n exact repetition of properties. It 
is clear that a very close analogy exists between this 
theoretical series of atoms of Thomson's and the actual 
system of the elements, as revealed in the periodic table. 
The first member in each " period" in the table (see Sec- 
tion 6) may be supposed to coincide with the formation 
of a new ring, which would contain, in the given element, 
only one electron. 



As stated in Part I, the recent "nucleus theory" of 
the structure of the atom supposes that the positive elec- 
tricity, instead of forming a large sphere, coextensive 
with the general volume of the atom, is concentrated 
in a very minute central region. However, we still hear 
of concentric rings, or shells, of electrons surrounding 
this nucleus, and it is probable that, in a general way, the 
arrangement of the electrons resembles that in the 
Thomsonian atom. 

The Nucleus Theory. The " nucleus theory" was 
proposed by Rutherford to explain the manner in which 
the alpha rays from radio-active substances are scat- 
tered as a result of their passage through material bodies. 
This scattering may be supposed to be caused by the ac- 
tion of the intrinsic forces of the atoms of the body upon 
the charged particles which make up the rays. The 
degree of scattering is measured by the angle made by 
the path of the ray as it leaves the thin sheet of substance, 
through which it has passed, and its original line of 
travel. Now experiment shows that even when the 
average degree of scattering is relatively low, a small 
fraction of the rays are turned through a very large angle, 
often so that their motion is actually reversed in direction. 

Since the original speed of the alpha particles and their 
charge are known, it is possible to calculate what con- 
ditions would be necessary to cause a deflection of this 
magnitude. In the case of the element gold, such calcu- 
lations show that the positive electricity of the atom 
must be concentrated on a sphere about one trillionth of 
an inch in diameter, which is only one ten-thousandth 
part of the diameter of the atom itself. 

It is estimated that in passing through hydrogen gas, 
some of the alpha ray particles come within one twenty- 
five-trillionth of an inch of the centers of the positive 



nuclei. Since this is less than the diameter of an electron 
it seems probable that the bare positive nucleus is smaller 
than are the negative particles hi the atom. In Section 
25 we have seen that on the assumption that the electron 
is made of pure negative electricity, it is possible from a 
knowledge of its mass and charge to calculate its size. 
Now, the facts indicate that the mass (or weight) of the 
positive component of the atom is enormously greater 
than that of the electron, so much so that it is practically 
equivalent to the total weight of the atom. On this basis, 
calculation shows that the nucleus of the hydrogen atom 
if its mass is wholly electrical must have a diameter 
of about one ten-quadrillionth of an inch, or one eighteen- 
hundredth that of the electron. This result appears to 
be in harmony with that reached by the study of the alpha- 
ray scattering. 

The supposition that the positive components of the 
nucleus of most atoms have electrons closely bound up 
with them is necessitated by the facts of radio-activity. 
The tremendous speed with which the electrons of the 
beta rays are sent off, demands original intra-atomic 
forces which could only result from an exceedingly close 
packing together of the atomic parts which are involved. 
Moreover, there seems little room for doubt that there 
are at least two classes of electrons in the atom, (1) those 
which directly determine its physical or chemical proper- 
ties, and a fraction of which can be separated from the 
atom and replaced again without great difficulty, and (2) 
those which leave the atom only during a radio-active 
change, and the loss of which means an apparently 
irrevocable alteration in the fundamental nature of the 

It seems probable, however, that electron groups of 
varying degrees of superficiality, so to speak, exist within 



complex atoms. These may be thought of as correspond- 
ing with the successive rings of the Thomsonian atom. 
The most superficial system of all is that of the so-called 
valency electrons, which are probably responsible for the 
more obvious chemical properties of the substance and, 
as Stark believes, for its band spectra. Still deeper 
electron layers give rise to the recently discovered X ray 
spectra (see Section 55). 

The Number of Electrons in the Atom. Numerous 
attempts have been made to calculate the number of 
electrons in the atoms of the various elements. This 
can be done on the basis of the degree hi which rays of 
several sorts are scattered in passing through sheets of the 
elements in question. The more electrons there are- 
that is, obstacles for the rays to encounter the more 
scattering will occur. Results obtained by a number of 
different methods indicated that the number of electrons 
is approximately equal to one-half the atomic weight. 
But it is clear that if, as in the case of hydrogen, one 
unit of positive charge is always associated with one unit 
of weight, the number of electrons in the atom would 
have to be accurately equal to the atomic weight in order 
to balance the charge of the nucleus. We are led, there- 
fore, to suppose that there are as many electrons bound 
up in the nucleus as there are in the body of the atom. 
The number of these latter, " external" electrons is con- 
trolled by the magnitude of the unbalanced charge of the 
nucleus, which is represented by the "atomic number" 
(vide infra). As a matter of fact, as an examination 
of Table I, Section 5, will show, the nuclei of the 
heavier elements must contain more electrons than the 
body of the atom. 

Isotopism. Radio-activity, as we have stated, is 
indubitably an affair of the nucleus, and it appears to 



depend upon the ejection either of a doubly, positively, 
charged helium atom alpha particle or a singly, 
negatively, charged electron beta particle. It is ob- 
vious that if the nucleus loses one alpha particle and 
then two electrons its resultant charge will be the same 
as before these three radio-active changes had occurred. 
Consequently, the number and arrangement of its exter- 
nal electrons will be the same as before the change, and 
the two elements, although differing in atomic weight 
by four units, will have identical chemical and physical 
properties. As shown in Table II, Section 5, many such so- 
called isotopes have been demonstrated among the radio- 
active elements. A mixture of isotopes acts like and 
indeed is a chemically pure substance, and there is no 
chemical reaction which can be used to separate its 
components. However, these components can easily be 
distinguished from each other by radio-active tests. 
Chemical tests distinguish between only ten kmds of 
radio-elements, whereas radio-active tests show thirty- 
four or more. 

Atomic Numbers. The expulsion of an alpha particle 
from an atom causes the corresponding element to move 
two places from right to left in the periodic table (see 
Section 6); the loss of a beta particle reverses this move- 
ment one place. It appears, then, that the position of an 
element in the table depends not upon its atomic weight, 
directly, but upon the resultant positive charge of its 
nucleus, which is called its atomic number, and the 
successive places in the table correspond with unit dif- 
ferences in this charge. Consequently, in so far as the 
table is regarded as a chemical schema, its principle should 
be restated as follows: "All of the chemical properties 
of the elements are periodic functions of their atomic 
numbers (instead of weights)." Chemical analysis, it 


would appear, is really an analysis of matter only into 
different types, not into different elements. 

The recent discovery of a means of measuring the wave- 
lengths of X rays has shown that all of the elements, 
under the right conditions, give off X rays, the lengths 
of which are characteristic of the element in question. 
It has been found possible to calculate these wave-lengths 
by means of a very simple formula involving the atomic 
number or, vice versa, to deduce the atomic number from 
the wave-length. Measurements of this sort by Moseley 
indicate that the atomic number of gold is 79, and that 
from aluminium to gold, in the periodic table, only three 
possible elements are missing. The atomic number of 
the heaviest element, uranium, appears to be 92, although 
there is some disagreement concerning this point. If this 
is correct and if uranium is the heaviest element which 
can exist, then it means that only 92 different chemical 
elements are possible. 

The Hydrogen Atom. In some respects hydrogen 
occupies a unique position hi the system of the elements. 
The alpha ray particle, which is a helium atom with a 
double positive charge, may be thought of as consisting 
of the helium nucleus stripped of its external electrons. 
The ordinary hydrogen ion, bearing one positive charge, is 
probably the bare nucleus of the hydrogen atom. How- 
ever, the alpha particle contains four units of weight to 
two of charge, while the hydrogen ion has one unit of 
weight to one of charge. On the basis of the argument 
previously presented it would appear that while the helium 
nucleus has bound up with it two electrons, the hydrogen 
nucleus, or ion, consists of pure positive electricity, and 
is, hi fact, a positive electron. 

On the supposition that the uncharged hydrogen atom 
is thus made up of two electrical particles, one positive 



and the other negative, the Dutch physicist, Bohr, has 
shown that by use of the known electro-magnetic laws 
and the new "quantum" theory of light (see Section 54), 
some very remarkable conclusions can be reached. For 
a long time it has been known that the position of the 
" lines" in the hydrogen spectrum can be calculated with 
amazing exactness by means of a certain mathematical 
formula (named after its discoverer, Balmer) which had 
been arrived at empirically by a method of trial and error. 
Up to the recent work of Bohr, however, it seemed im- 
possible to derive this formula by means of the laws of 
simple mechanics and electricity from any conceivable 
hypothesis about the structure of an atom, especially 
from a simple one. But by introducing the assumptions 
of the new theory of light, Bohr has shown that the for- 
mula in question follows very simply from the conception 
of the hydrogen atom above described. This result can 
hardly be considered other than epoch-making in the 
history of atomic and optical theory. 

Bohr's theory disposes of a difficulty which has bothered 
physicists for some time, u/z., the problem as to why the 
electrons within the atom continue to rotate about the 
center of attraction, instead of rapidly falling into it. 
Centrifugal force would keep them out as long as they 
maintained their speeds, but the ordinary laws of radia- 
tion demand that these speeds should constantly dimin- 
ish, owing to the continuous emission of energy in the 
form of electrical waves. However, the quantum theory 
necessitates that such emission should occur only in 
discontinuous units of fixed magnitude. Consequently, 
it would be impossible for the electron to drop gradually 
into the atom; it would be obliged to fall in steps or jerks. 
Each jerk would generate a characteristic wave, corre- 
sponding with a definite line in the spectrum of the ele- 



ment it being consistent with the facts to suppose that 
line spectra are formed only when electrons are separated 
from or returned to a definite place in the atom. When 
the electron has completed any drop and radiated the 
corresponding energy it remains in equilibrium, probably 
rotating about the nucleus, but no longer radiating. 

It is difficult to handle mathematically the problems 
connected with the exact structure of atoms more com- 
plex than that of hydrogen, but it seems probable that 
these atoms are made up of larger numbers of positive 
particles, and electrons, rotating in more complex ways. 
It is apparent that there is a close analogy between the 
structure and internal processes of atoms, as conceived 
in modern theory, and those of astronomic systems. 
This is why the type of atom above considered is some- 
times characterized as the "Saturnian atom." 


J. J. Thomson's theory of atomic structure is well summarized 
in popular form by R. K. Duncan in "The New Knowledge " (1908), 
Part 5, Chapter II. Thomson's own account appears in popular 
form in his "Electricity and Matter" (1904), Chapter V. 

See also Norman Campbell's "Modern Electrical Theory," 
Second edition (1913), Chapter XIII. Bohr's articles are in the 
Philosophical Magazine for July, September, and November, 1913, 
Vol. 26, pp. 1, 476 and 857. J. J. Thomson's latest theory will 
be found in the same journal for October, 1913, p. 792. 

On isotopes see Frederick Soddy's "The Chemistry of the 
Radio-Elements" (1915), Part II. On atomic numbers and the 
Bohr atom: W. H. and W. L. Bragg's " X Rays and Crystal Struc- 
ture" (1915), pp. 77 to 87. 

Section 54 

The Nature of the Theory. When light travels from 
one part of space to another there is a motion of a certain 



amount of "radiant energy" through the intervening dis- 
tance, and at any time during the motion this energy 
must be localized in a definite region of space. The ques- 
tion as to whether radiant energy is or is not atomic may 
consequently be asked in the following way. Is light 
energy travelling in free space spread out uniformly and 
continuously in that space, or is it concentrated hi a 
limited number of relatively small and clearly defined 
regions, the amount and intensity of the energy in each 
of these regions being invariable for a given kind of 

The first alternative is the one which had been accepted 
up to quite recent times. It assumed that radiant energy 
can be emitted from bodies continuously, in any amount 
and at any intensity, that it spreads out uniformly from 
its source like a perfect non-molecular fluid, becoming 
steadily weaker the further it goes from the source, ac- 
cording to the well-known " law of inverse squares." 
The second alternative, which corresponds to the modern 
doctrine of light " quanta," denies all of these assump- 
tions. It states that light is not radiated from bodies 
continuously, but instead in sudden outbursts, each of 
which is of definite magnitude and intensity determined 
only by its wave-length or frequency. It would be im- 
possible for a body to radiate a fraction of one of these 
units, so that quantities of radiation which are not inte- 
gral multiples of the units in question cannot exist. 

Moreover, the intensity of one of these light atoms, or 
quanta, does not decrease with its distance from the 
source, and consequently it does not spread out as it 
travels. The reason that light seems to fall off in intensity 
with distance lies in the fact that the number of light 
atoms to be encountered in a given region of space natu- 
rally becomes less the farther that region is from the emit- 



ting body. This conception of the structure of a beam of 
light means that all optical images are really built up on 
the same principle as the ordinary " half -tone" engrav- 
ing, that is, they are made of minute dottings or stipplings 
far too small to be detected by the eye. (However, the 
sensitiveness of the retina is so great that a visual sensa- 
tion can be produced by relatively few quanta of the 
right kind of light.) 

Such a striking alteration as this in the theory of light 
cannot be without strong grounds. In discussing these, 
no attempt will be made to follow the order of their his- 
torical appearance in connection with the problem. 

The Photo-Electric Effect. We have seen in Chapter 
V, that many solid bodies, especially metals, under the 
influence of high temperatures, give off electrons. It 
has been found that they also can be made to emit elec- 
trons at ordinary temperatures if their surfaces are 
exposed to the action of ultra-violet light or X rays. If 
we are not to assume that the metal becomes radio-active 
under the action of the light, we must suppose that the 
energy of motion of these emitted electrons comes from 
the light itself. The very curious fact now appears that 
this energy that is, the highest speed with which any 
of the electrons travel is independent of the intensity 
of the light which shines upon the surface. If the light 
is weak, relatively only a few electrons are given off, 
but those which do appear have the same velocity which 
is characteristic of the effect with lights of higher intensity. 
These results seem to be compatible only with some such 
notion of the atomic structure of light as we have just 

Measurements upon the speeds of the electrons in the 
photo-electric effect as the phenomenon is called 
prove that their energy of motion, although independent 



of the intensity, is closely proportional to the "frequency" 
of the rays employed, that is, the shorter the "wave- 
length " of the radiation the faster the emitted electrons 
move. This connection between "frequency" and energy 
is one of the fundamental principles of the quantum 
theory; it resolves itself ultimately into the statement 
that the energy of any light quantum is directly propor- 
tional to its frequency. The higher the frequency the 
higher the energy, but for a given frequency the energy 
is fixed and invariable. 

If this principle is valid, it is clear that it should be im- 
possible to generate quanta of high frequency from those 
of low frequency, since this would contradict the law of 
the conservation of energy. In accordance with this, it 
has been shown experimentally that when one kind of 
radiation is changed into another as, for example, 
" fluoresence " and "phosphorescence" the alteration 
in wave-lengths is, in most cases, from high to low 
frequency. This empirical principle is known as Stores' 
law. It holds for the transformation of X rays, as well 
as for those of ordinary light. 

Such transformations as these must always be accom- 
plished by permitting the light to fall upon some material 
body, and there is practically no doubt that the active 
factors in the change are the electrons which are bound 
up in the atoms of the body. Experiment makes it prob- 
able that all such electrons have natural rates of vibration, 
which depend upon the constitution of the atoms or mole- 
cules of which they form parts. The photo-electric effect 
and its analogues in all probability depend upon the 
ejection of electrons from atoms under the influence of 
the electrical forces in the light ray. The quantum theory, 
however, makes it seem probable that (1) the energy of 
the light cannot be transferred to the electron unless the 



frequency of the light is at least approximately the same 
as that natural for the electron itself, and (2) if the elec- 
tron takes up any of the energy of the light quantum it 
must take it all. In harmony with this view it is found 
that so far as present measurements permit us to judge, 
the energy of the electrons emitted under the action of 
light and X rays is the same as that of the respective light 

Other Fads Underlying the Theory. There are fur- 
ther important results of the idea that an electron, atom, 
or molecule can take on or part with only whole light 
quanta, and only such quanta as have approximately their 
own natural frequencies. For example, it can be shown 
theoretically, upon certain reasonable assumptions with 
regard to the molecular conditions underlying the facts 
of specific heat, that if the quantum theory is true the 
specific heats of all substances at or near absolute zero, 
should themselves be very close to zero. At low tem- 
peratures the average energy of vibration of the atoms is 
so small that only a few of them are able to retain whole 
quanta of energy, and if they are unable to retain whole 
quanta they cannot have any energy at all. Consequently, 
at low temperatures the atoms of a body lose their power 
to absorb heat, and hence the body suffers a radical de- 
cline in its specific heat. (See Section 24.) The recent 
experimental work of Nernst and his co-workers has 
shown that such changes in the specific heats of bodies 
actually do occur, and that their manner of occur- 
rence satisfies expectations based upon the quantum 

However, probably the most important consideration of 
all which is presented at some length in Chapter X, 
and which, unfortunately, is too involved to be developed 
much more completely here is that which first led the 



German physicist, Max Planck, to propound the quantum 
theory. It concerns the manner in which the amounts of 
light of different wave-lengths emitted by glowing solid 
bodies at various temperatures are related with the wave- 
lengths and temperatures in question. This relation is 
represented in the " curve of distribution" of energy in 
the spectrum, which has been discussed hi Section 39. 
For any given temperature there is a certain wave-length 
which has a higher intensity than any other. Wave- 
lengths greater or less than this have increasingly lower 
intensities. Various only partially successful attempts 
had been made by several physicists to explain the exact 
relations between temperature, wave-length, and inten- 
sity in terms of the electron theory of radiation. Planck 
found that such an explanation could be given if it be 
assumed that the radiation takes place discontinuously, 
and that each quantum of light radiated has an energy 
proportional to its frequency, i.e., inversely proportional 
to its wave-length. The demonstration of the possibility 
of such an explanation proved to be of epoch-making 
importance in theoretical physics. 

Significance of the Quantum Theory. The possible far- 
reaching significance of these developments in the theory 
of radiation may perhaps be suggested by a few state- 
ments, the whole meaning of which, however, can only 
be appreciated by one closely acquainted with physical 
science and its history. In the first place, it has been 
shown conclusively by the English physicist, Jeans, that 
although the facts just mentioned as Planck's first basis 
for the quantum theory demand an atomic view of the 
nature of radiation, the interpretation given to them 
makes them wholly inconsistent with the most funda- 
mental principles of the science of mechanics. In other 
words, it would appear that, to a certain extent at least, 



events in the world of atoms and electrons do not follow 
the laws of ordinary mechanics. 

Secondly, it appears that these events do involve in a 
very definite way certain considerations based upon the 
theory of probabilities or chance. For some time it has 
been known that the so-called second law of thermody- 
namics, which states that the amount of available energy 
in the universe tends constantly to decrease, could be 
derived from a study of the relative probabilities of given 
configurations of the molecules composing any group of 
bodies. Certain arrangements of the molecules are more 
probable as the outcome of a disturbance than are others. 
There would always be a tendency for the less probable 
configurations to be replaced by the more probable ones. 
This tendency corresponds with the statement that the 
" entropy" of a given group of bodies tends always to 
increase; the greater the entropy of the group the less 
available energy there is in it, the nearer all of energy 
in the system is to being uiiiformly distributed heat. 
This latter state of affairs is the most " probable" of all 
states of the molecular system, and the " entropy" of the 
system as a whole is merely another expression for the 
probability of the particular configuration of molecules 
or molecular conditions existing within it. 

In order that considerations of this sort, involving the 
doctrine of chance, should be applicable to a subject 
matter, it is absolutely necessary that this subject matter 
consist of discrete individuals or particles, in short that 
it be atomic. Now, it has been shown very decisively 
that the conception of entropy and the principles of thermo- 
dynamics at large are definitely applicable to the be- 
havior of radiant energy, and hence we must almost 
inevitably conclude that such energy is atomic hi nature. 

It is of course quite possible that radiant energy is 


atomic while other forms of energy are not, that there 
will be a limit to the application of the principles of 
atomism to the physical universe. Indeed, certain well- 
known physicists still hold that even the facts which 
support the quantum theory of light can be explained 
without any radical change in our present doctrines. The 
exact outcome of this contention still rests in the balance. 


A remarkably complete, although somewhat mathematical dis- 
cussion of the Quantum Theory and its grounds is given by Norman 
Campbell in his "Modern Electrical Theory," second edition 
(1913), Chapter X. A somewhat simpler, but also very clear ac- 
count is that by R. A. Millikan, "Atomic Theories of Radiation," 
in the journal Science for January 24, 1913 (Vol. 37, pp. 119-133). 
For a popular discussion of the growth of atomic theories in phys- 
ics see Sir Oliver Lodge's "Continuity" (1914). 

Section 55 

The Origin and Nature of X Rays. X rays are formed 
when the cathode rays, of which we have spoken in 
Section 25, are stopped hi their course by striking a solid 
plate, commonly called the " anti-cathode." Since the 
electrons of the cathode rays are moving much more 
slowly than are those of the beta rays (see Section 49), 
the " kinks" (see Section 37) of which they are made up 
are not so sharp, but they are nevertheless very much 
sharper than those of ordinary light. 

As mentioned incidentally hi Section 53, it has been 
shown that a fraction of the X rays given off by the anti- 
cathode have a wave-length and penetrative power which 
is characteristic of the element of which the anti-cathode 
is composed. These " characteristic X rays" are of 


X RAYS [Sec. 55 

shorter wave-length the higher the atomic weight of the 
element, and are wholly independent of its state of 
chemical combination. Most of the elements give out 
characteristic X rays of two different wave-lengths, and 
when these are analyzed by an X ray spectrometer, they 
form the "X ray spectrum" of the element in question. 

Why X Rays Penetrate "Opaque" Bodies. It is the 
extreme "sharpness," or very high frequency, of the 
gamma and X rays which chiefly distinguishes them 
from ordinary light, and which gives them their special 
power to penetrate solid bodies which are opaque to light. 
Bodies absorb light only because their atoms are able to 
respond, or " resonate," to the vibrations in the light 
wave (see Section 41). But when these vibrations are 
very rapid or, what means the same thing, when the 
"kinks" are very short the atoms of the substance can- 
not respond with sufficient quickness, and hence the light 
is not absorbed. There is some question as to whether 
this explanation holds exactly on the quantum theory of 
radiation. However, there is doubtless also a specific 
relation between frequency, as such, and degree of ab- 
sorption, since certain bodies strongly absorb X rays of 
one frequency and not of others. 

The Corpuscular Properties of X Rays. Certain phys- 
icists, among them W. H. Bragg, formerly supported the 
view that the gamma and the X rays are not waves but 
are moving particles. The reason for -this advocacy lay 
hi the fact that these rays, in passing through matter, 
behave as if their energy were concentrated in very mi- 
nute, moving regions, instead of being spread out over a 
continuous " wave-front," as light energy is supposed to 
be in the classical theory. However, as we have seen 
in Section 54, latter-day developments have proved that 
ordinary light has the same sort of distribution, so that 



the evidence in question counts no more against the wave- 
theory of X rays than it does against that of light in gen- 
eral. Recent developments, briefly discussed in Part I, 
have completely converted Bragg and his school to the 
wave theory or at least to the general theory of radiation. 
The Reflection and "Diffraction" of X Rays by Crystals. 
The reflection of X rays from a crystal surface is 
somewhat different from that of ordinary light. The 
reason for this is to be found in the fact that although the 
atoms of the crystal are regularly arranged, they are still 
so far apart compared with the wave-length of the rays 
that the discontinuities produce a sensible effect. In 
Chapter VI of Part I the reflection of ordinary light has 
been explained as due to the generation of a return wave 
by the electrons of the atoms hi the surface of a body 
which is struck by light. This is what occurs also in the 
case of X rays, but owing to their penetrative power and 
relatively short wave-length the return waves from dif- 
ferent planes of atoms in the crystal do not fuse with 
each other harmoniously, except hi certain favorable 

In other directions there is interference, that is, the 
waves from one layer of atoms oppose those from another 
layer and wipe them out. In fact, the X rays are so short 
that the atoms of the crystal form for them a "diffrac- 
tion grating," similar in action to the mechanically ruled 
gratings used to bring about the interference of ordinary 
light waves. 

Now, the direction in which interference does not take 
place in the case of X ray reflection depends upon the 
wave-length of the rays. Consequently, if we measure 
the angle at which the reflection of a given set of rays 
occurs most readily, and if we know the structure of the 
crystal, we can calculate the wave-length. Conversely, 



[Sec. 55 

using rays of known wave-length, we can deduce the 
structure of a crystal with the constitution of which we 
are not familiar. (See Figure 27.) 

Q ------- 

Fig. 27 

This drawing represents the structure of a crystal of potassium chloride, 
a substance similar to ordinary salt, as deduced from its action upon X rays. 
The dark spheres represent chlorine atoms, the light ones atoms of po- 
tassium. It will be seen that the unit of structure of the crystal is the 
individual atom, since all of the atoms are equidistant from their imme- 
diate neighbors. For the sake of clearness, the spaces between the atoms 
have been exaggerated, as compared with their diameters. 

There are always a number of different ways in which 
geometrical plane surfaces can be drawn through the 
atoms in a crystal, and each of these theoretical surfaces 
is capable of reflecting or diffracting X rays. The result 
is that if a beam of rays is sent into a crystal, it is partly 

[ 192 ] 


split up into secondary beams which take different direc- 
tions, characteristic of the inherent planes of the crystal 
atoms. When these latter beams are caught upon a 
photographic plate a pattern is produced from which the 
constitution of the crystal can be inferred. 


Concerning the X rays see: C. W. C. Kaye's "X Rays" (1914); 
and W. H. and W. L. Bragg's "X Rays and Crystal Structure," 
(1915). Also: S. P. Thompson's "Radiation" (1898), Chapter III. 
Bragg's arguments with reference to the corpuscular properties of 
the rays are given in Norman Campbell's "Modern Electrical 
Theory," second edition (1913), pp. 292-304. 

Section 56 

Living bodies are complex mixtures of active chemical 
substances. These substances are constantly reacting 
with each other and as a consequence the bodies in 
question would soon be destroyed if it were not for the 
fact that the chemical changes are so organized and con- 
trolled as to ultimately bring compensation for the de- 
struction which they cause. 

This control and regulation which is so characteristic 
of the activities of living beings probably depends in the 
last analysis upon a purely chemical principle called 
catalysis. This principle implies that the mere presence, 
in chemical mixtures, of very minute quantities of cer- 
tain substances, can determine the nature of the changes 
which take place in these mixtures. Controlling sub- 
stances of this sort are called catalyzers. The effects 
which they produce can be explained in terms of atoms, 
molecules and electrons. 



The catalyzers which control the life processes in dif- 
ferent organisms are characteristic of these organisms, 
and are undoubtedly transmitted to them from their 
progenitors through the germ-cells from which they 
originally developed. The reason why certain catalyzers 
and not others have been constantly transmitted from 
parent to offspring through many generations is to be found 
in their special power to regulate the chemical changes in 
organisms so as to permit the survival of the species. In 
other words, the present physical and chemical structure 
of organism must be explained not only in terms of atoms 
and molecules but also in terms of the history of living 
matter upon the earth. 

The most important elements hi the constitution of 
living organisms are carbon, hydrogen, oxygen, and 
nitrogen, although many others are essential. These 
elements are combined, usually, to form "colloidal" 
systems of particles (see Section 3), and many of the 
fundamental peculiarities of living things depend upon 
those of colloids. 

Organic catalyzers are called enzymes, and on the 
above theory, enzyme action explains the mystery of 


See L. T. Troland's "The Chemical Origin and Regulation of 
Life," in the Monist for January, 1914. 




NOTE: Page numbers in italics refer to the more extensive 
discussions of a given subject. 

Absolute zero, 20, 107 

, no chemical action at, 29 

, state of atoms at, 55 

Absorption of light, 30 
jEther, present status of, 146 
Alcohol, formula of, 80 
Allotropism, 84, 88 
Alpha rays, 34, 35, 168 

, counting particles in, 56 

, origin of, 45 

, proved by Rutherford to be 

helium atoms, 169-170 

, scattering of, 67, 176 

Argon, 138 

Aston, F. W., on meta-neon, 75 

Atom, energy of, 772 

, internal forces of, 10 

, nucleus, size of, 176-177 

, nucleus theory of, 44-45, 71, 74, 


, openwork structure of, 42 
, permanence of, 10 
, Saturnian, 61, 182 

, radiation from, 181-182 

, single, effect due to, 35 

, solar system theory of, 42 

, structure of, 41-43, 43-46, 174- 

782. (See Atomic structure) 
, Thomson's theory of structure, 


, unit of crystal structure, 113 
Atomic heats, 118 

magnitudes, 58 

- numbers, 46, 179-180 

structure, recent discoveries con- 

cerning, 43-46 
and spectra, 155 

Atomic volumes, 3-4 

and atomic weights, 67 

weights and atomic numbers, 46 
, irregularity of, 75 

, methods of determining, 65 

, table of, 62-3 

Atoms, 2 

and electrons, relations between, 


and life, 50-51 

and molecules, relative sizes of, 

(Fig. 1), 3 

and positive electricity, 23 

, arrangement in molecule, 76-86 
, attraction of identical, 138 

- neutral, (Fig. 22), 138 
, charges carried by, 24 
, density of, 4 
, individuality of, 5 
, kinds of, 2-3 
, number in unit volume, 54, 56 

of a liquid, (Fig. 5), opposite page 


of a solid, (Fig. 7), opposite page 


of light, 183 

, shape of, 2, 60-61 
, sizes of, 2 

, methods of finding, 53-58 

, species of, 62 

, spontaneous disruption of, 10 

, structures of, and periodic table, 


, tendency to form groups, 5 
, visibility of, 58-59 
Attraction, forces of, within bodies, 

Avogadro, principle of, 66, 107 




Balmer's formula, 181 
Battery, electric, action of, 25 
Becquerel, Henri, 52, 165 
Benzene, derivatives of, (Fig. 13), 


, ring formula of, 80-83 
Beta rays, 34, 168 
- and X rays, 189 
, from potassium, 173 

, origin of, 45, 177 

, penetrating power of, 35, 42, 

, relation to gamma rays, 38, 


, scattering of, 67 
Black body, radiation from, 152 
Bohr's theory of atomic structure, 

Boiling points, 105 

, effect of ionization on, 140- 


Boyle, law of, 106 
Bragg, W. H., on corpuscular prop- 
erties of X rays, 190-191 
Brownian movement, 16-17, 110- 

, path of particle in, (Fig. 16), 


Campbell, Norman, on radio-activ- 
ity of potassium, 173 

Carbon, allotropic forms of, 85 

, compounds of, 77 

Catalysis, 145 

- and life, 793-794 

Cathode rays, 120 

, action of, (Fig. 18), 121 

, action of magnet on, (Fig. 
19), 122, 163 
and X rays, 189 

Cell, electric, 145 

Cells, 149 

and atoms, 50-51 

Chances, molecular, and averages, 


Charles, law of, 16, 106 
Chemical affinity, 91, 736-739 

action, 7 

and electrons, 27-28 

Chemical affinity and ionization, 

, molecular basis of, 142-144 

change, effects and conditions of, 

, mystery of, 88 

energy, 146 

, in relation to radio-activity, 


formulae, 77 

properties of atom, determina- 

tion of, 45, 89 

reactions, reversibility of, 143 
, velocity of, 143 

valency, 141-142 
Chlorine, atomic weight of, 65 
Coagulation, 59 

Cohesion, forces of, 11, 92, 172 

Cold, nature of, 20 

Colloids, 59, 112 

- and life, 194 

Color, due to reflection, 158 

mixture, and white light, 158 
, physical basis of, 87, 757-759 
, sensations of, 158 
Compounds, 6 

, properties of, 86 
Conduction, electrical. (See Elec- 
trical conduction) 

of heat. (See Heat conduc- 

Conductivity, electrical, basis of, 


, of gases, 98 
Cooling, due to evaporation, 19 
Copper, color of compounds of, 88 
Critical points of liquids, 105 
Crystal, as a unit of structure, 112 
, fixity of molecules in, 103 
, planes of, 192-193 
, structural plan of simple, (Fig. 

27), 192 
structure and molecular form, 

and X rays, 49, 53, 773, 114, 


Crystalline state, 112 
Crystals, liquid, 19, 112, 114 
Curie, M. and Mme., 165 
Current, electrical. (See Electrical 




Dalton, John, 52 

Diamond, 88 

Dielectric capacity and electrolytic 

dissociation, 139-140 
and index of refraction, 160- 

Diffraction, 191-192 

of X rays, 113, 191 
Diffusion, 98, 99-101 

and atomic size, 57 

paths, (Fig. 16), 100 
Dispersion, of colloids, 59 

of light, laws governing, 160 
Distribution curve of temperature 

radiation, (Fig. 25), 153, 757- 

Du Long and Petit, on atomic heats, 

Dynamo, action of, 25, 33 

Elasticity, 87 

Electrical conduction, in gases, 737- 

, in liquids, 131-132 

current, direction of, 131 
, effects connected with, 729- 


, nature of, 24 

, produced by chemical 
change, 145 

force lines, kinks in, 147 

, nature of, 146-147 

forces, importance in nature, 724 

lamp, principle of, 25 

motor, principle of, 32 

power transmission, 26, 733 

resistance, nature of, 129, 130 

waves, absorption of, 30 
, generation of, 30 

, reflection of, 31 
Electricity, conduction in gases, 93 
, conduction through liquids, 140 
, in all bodies, 23 
, laws of attraction and repulsion, 


, positive and negative, 22 
, positive, form in Thomsonian 

atom, 175 

Electricity, relation to magnetism, 


, " speed " of, 24, 25 
Electrolysis, 737-732, 139-141 
, deposition by, 55 
Electrolytic dissociation, 739-747 
Electron, and its behavior, 27-27 
, charge of, 23, 120, 122, 123 
, contains only negative elec- 
tricity, 124 
, density of, 22 
, discovery of, 52, 720 
, flattening at high speed, 124 
, mass of, 120, 123 
, measurement of, 120-124 
, natural unit of electricity, 55 
, radiation from, 46 
, shape of, 22, 124 
, size of, 21, 123 
, structure of, 22, 724 
, weight of, 22 
, Zeeman effect, 29, 149 
Electrons, 2 

, action of magnetic field on, 129 
, affinity of atoms for, 736-739 
, affinity of elements for, 26 
and chemical action, 27, 28, 29 

and ions, reactions between, 


and light waves, 29, 147 

and line spectra, 155 

and magnetism, 32-34, 164 

and selective absorption of light, 


and temperature radiation, 150- 


and valency, 142 

, arrangement in atom, 175 

, as beta rays, 34-35 

, emitted by metals under action 

of light, 184 

, evaporation of, 27, 134 
, " free," 26, 130 

, and heat conduction, 110 
, in atom, 42-46, 175, 178 

, two groups of, 177 

, in the electric current, 24 
, moving, deflection by magnet- 
ism, 32, 762-763 
, number in atom, 178 
, position in atom, 44, 45. 



Electrons, rings of, in atom, 175 

, valency, 89, 177 

Elements, 5, 6 

, affinity for electrons, 26 

, chemical, as atomic mixtures, 74 

, number possible, 180 

, electro-positive and negative, 

26, 136-138 
, evolution of, 41 
, inert, explanation of, 138-139 
, life of, 41 
, molecules of, 83 
, periodic table of, 68-76 
, primitive, in hottest stars, 174 
, properties of, and periodic table, 

, radio-active series of, (Fig. 11), 


, radio-activity, alleged of all, 40 
, specific nature of, 62-63 
, systematic relations of, 68 
, table of, 62-63 
, undiscovered, 70 
Emulsions, 59 
Energy, 2 

, atomic nature of, 48 
, chemical, 146 

, equipartition of, 95-96, 111, 118 
, intra-atomic, 39, 172 
, kinetic, 95 
, given off by radium, 40 
Entropy, 188 
Enzymes, 194 
Equations, chemical, 90 
Equilibrium, chemical, 143-144 
Equipartition of energy, 95-96, 111, 


Ether, 29 
, methyl, 78 
Evaporation, 13 
, cooling effect of, 19 
Evolution, inorganic, Lockyer's 

theories, 173-174 
, organic, 194 
Expansion, due to heat, 18 

Fluids, motion of particles through, 


Fluorescence, 185 
Fog, condensation around ions, 122, 

123, 126 

Forces, inter-atomic, 91 
, intra-atomic, 91 
, magnetic, direction of, around 

moving electric charge, (Fig. 

26), 162 

Formulae, chemical, 77-75 
, graphical, meaning of, 78 
, structural, 77 
Free electrons, 26 

and electrical conductivity, 


and heat conduction, 110 

Frequency, and energy of light 

quantum, 185 
Freezing points, 87 

, effect of ionization on, 140- 


Friction, causes heat, 19 
, molecular explanation of, 12 
Fusion, latent heat of, 103 

Gamma rays, 34, 35, 36, 168 

, nature of, / 70-1 71 

, origin of, 45 
, relation to beta rays, 38 
, wave-length of, 156 
Gas law, 108 
, model of a, 14-16 
pressure, cause of, 15, 16 
Gases, atomic heats of, 119 
, conduction of electricity through, 


, emission of light by, 152, 153 
, simple laws of, 106-108 
Gay-Lussac, principle of, 107 
Geiger and Nuttall, on life of radio- 
elements, 167 
Gold-leaf, molecular thickness of, 

Gravitation, 91 


Families, in periodic table, 69 
Faraday, Michael, 52 
Films, thin, 54 

Hall effect, 129 
Hardness, 87 


Heat and allied phenomena, II- 

and chemical action, 28, 145 

conduction, 16, 57, 109-110 

and electrical conduction, 130 

, due to electric current, 25 

, due to friction, 19 

energy, 118 

in bodies, amount of, 20-21 

motion, visibility of, 16 
, radiant, 13, 29-30, 102 

, produced by chemical change, 

wave, 156 
Heats, latent, 102-105 
Helium, 138 

, in alpha rays, 35 

, Rutherford's experi- 
ment, 170 

, in atom structure, 71 

, in stars, 174 

Heredity, 194 

Hertz, H. R., 52 

Hertzian waves, 29, 48, 156 

Hydrocarbons, five isomeric, (Fig. 
12), 79 

Hydrogen, 96 

f atom, structure of, 71, 74, 180- 

ion, nature of, 180 
Hysteresis, 165 

Index of refraction, 160 

Interference, of X rays, 113, 191 

Intra-atomic and inter-atomic 
forces, 91 

Inverse squares, law of, for radia- 
tion, 183 

lonization, 126 

, energy of, 126 

of substances dissolved in water, 

Ions, atoms and electrons, forces 
between, (Fig. 20), 127 

, conduction of electricity by, 

, how produced, 125-126 

, motion through solutions, 140 

Isomerism, stereo-, 84-86 

Isomers, and structural formulae, 


Isotopes, 45, 53, 71-75, 89 
, table of, 64 
Isotopism, 178-179 

Jeans, J. H., on quantum theory, 

Kinetic energy, 95 

Kinetic molecular theory, 92-9^, 

Kleeman, R. D., on atomic shapes, 


Latent heats, 102-105 

Laue, M., experiments on X rays, 


Lenard, P. A., 52 
Lenses and prisms, action of, 160 
Life and atoms, 50 

and catalysis, 59 

and colloids, 59 

Light, absorption of, 30, 157-158 

and chemical action, 28 

and electronic vibration, 29 

and magnetic field, 34, 149 

, conditions of production, 150- 

emission by gases, 152, 1 54 

frequency of, 157 

ionization due to, 126 

polarized, 86 

produced by chemical change, 

reflection of, 31, 157 

refraction of, 159-161 

selective absorption of, 157 

ultra-violet, 156 

velocity of, 156 

, in different media, 31 

, wave-length of, 156 

waves and electrical force-lines, 


Lines of force, electrical, 146-147 
Line spectra, 154 

and structure of the atom, 



Liquid, model of a, 17-18 

state, 13 

Liquids, conduction of electricity 

in, 131-132 
Lockyer, Sir Norman, on inorganic 

evolution, 173-174 
Loreutz, H. A., 52 

Magnetic field, due to motion of 

electricity, 161-162 
Magnetism, 163-165 
, dia- and para-, 163 
, permanent, 33, 164 
, relation to electricity, 32, 767- 


Magnetization, mechanism of, 164 
Malleability, and temperature, 103 
Mass action, law of, in chemistry, 

Matter, history of modern theory 

of, 52-53 

Maxwell, J. C., 52 
Mean free path (of molecules), 


and electrical conduc- 
tivity, 130-131 
, properties dependent on, 

Mechanics, fundamental laws of, 

92, 93 

, laws of, and atomic events, 787- 


, statistical, 93, 94, 97 
Melting, 13, 14 

points, 87 

Membrane, semi-permeable, 109 

Mendelejeff, and periodic table, 70 

Mercury vapor, 83-84 

Metals, heat conduction in, 110 

, thermo-electric series of, 135 

Meta-neon, 75 

Mica, 112-113 

Millikan, R. A., experiments of 

quantum theory, 48 
Molecular action, and probability, 

93, 94 

activities, individuality of, 94 

chaos, 142-143 

forces, internal, 86-87 

Molecular motion, speeds of, 94-97 

speeds, distribution curve of, 

(Fig. 17), 116 

, distribution law of, 775-777, 


theory, kinetic. (See Kinetic 

molecular theory) 

volumes, and gas law, 108 

weights, 66 

Molecule, arrangement of atoms 

in, 76-86 

, motion between impacts, 97-99 
Molecules, 5, 6 

and visible particles, relative 

sizes of, (Fig. 2), 4 
, electrical constitution of, (Fig. 

10), 28 

, frictionless, 12, 13 
, gas, motion of, 14 
, mean free path of, 97-99 
, motion of, 77-27 

of a gas, (Fig. 8), opposite page 


of steam, (Fig. 4), opposite page 


of water, (Fig. 3), opposite page 4 
, structure of, and crystals, 85 
Moseley, H. G. J., on atomic num- 
bers, 180 

Motion, laws of, 92, 93 
Motor, electric, principle of, 32 

Neon, and meta-neon (Isotopes), 75 

Nernst, W., on specific heats, 186 

Newton, laws of, 93 

Niton, 166 

Non-conductors, electrical, 130 

Oil films, 54 

Organic compounds, 76 

, formulae, (Fig. 6), 8-9 
Osmotic pressure, 108-109, 144 
Oxygen, atomic weight of, 66 

Periodic table of the elements, 68- 



Periodic table and atomic numbers, 

, blanks in, 70 

, defects in, 70 

, significance of, 70 

, Thomson's explanation of, 


Periods, in periodic table, 69 

, and electron rings, 


Perrin, J., on the Brownian move- 
ment, 110 

Phosphorescence, 185 

Photo-electric effect, 184 

Physical properties, basis of, 87 

Planck, Max, on quantum theory, 
46, 187 

Positive electricity, form in Thom- 
sonian atom, 175 

Potassium chloride, crystal struc- 
ture of, 113 

, radio-activity of, 40, 41, 773 

Power, electrical transmission of, 
26, 133 

Pressure, of a gas, 16, 106-107 

, osmotic, 108-109 

Prism, action of, on light, 155 

Probability and entropy, 188 

and molecular action, 93, 94 

Protyle, 71 

Prout's hypothesis, 71 

Pyrometer, optical, 151 

Quantum theory of radiant energy, 
46-49, 53, 147-148, 182-189 
and specific heats, 119-120, 

and hydrogen atom, 181 

and temperature radiation, 

152, 187 

, difficulties with, 48-49 
, significance of, 187 

Radiant energy, similarity of all 

forms, 49 
Radiation, due to acceleration of an 

electron, (Fig. 23), 148 
, due to temperature, 150-152 

Radiation, quality dependent on 

temperature, 47 
, recent discoveries concerning, 


Radio-active substances, 165-168 
Radio-activity, 34-41 

and atom structure, 44 
, cause of, 38 

, energy of, 39-40 

, general property of matter, 40, 


, laws of, 166, 167 
, not chemical, 39 
, penetrating power of rays, 124 
Radio-elements, 40 
, chemical diversity of, 179 
, discovery of, 165 
, isotopes among, 74 
, life of, 166 
, position of, in periodic table, 


, successive disruptions of, 38 
, table of, 64 
Radium, 34 
, atomic weight of, 40 

atom, stability of, 172 
, discovery of, 165 

, rays from, study of, 168-169 
Rayleigh, Lord, radiation law, 47 
Rays, alpha. (See Alpha rays) 
, beta. (See Beta rays) 
, cathode. (See Cathode rays) 
, gamma. (See Gamma rays) 
, X. (See X rays) 
Reactions, chemical, kinds of, 90 
Reflecting power, basis of, 158 
Reflection, diffuse, 31 

of light, 31 

of X rays, 49 
Refraction, index of, 160 

of light, 31 

, relation to absorption of light, 

Resistance, electrical, 98 

, nature of, 129-130 
Retina, the, 158 
Roentgen, W. C., 53 
Rowland's experiment, 161 
Rutherford, Ernest, 53 

, on helium in alpha rays, 769- 



Rutherford, Ernest, on scattering 
of alpha rays, 176 

Schmidt, G. K., 165 

Secondary rays, 171 

Series, disintegration, of elements, 

, thermo-electric, 135 

Soap-bubble films, 54 

Solid and crystalline states, 112- 

and liquid, stages between, 114 

, liquid, and gaseous states, 13 

, model of a, 18 

Solution, and electrical decomposi- 
tion, 139-141 

, simple laws of, 106-108 

Sound, 13, 101-102 

Specific heats, 118-120 

, at low temperatures, 119, 

Spectra, line, 755 

, varying complexity of, 173 

Spectral lines, 42, 43 

Spectrum, normal, 156 

Stark effect, 150 

Stark, J., 150 

Statistical mechanics, 93, 94 

Stokes' law, 185 

Stoney, G. J., 52 

Sugar, constitution of molecule, 

Sugars, formulae and crystals of, 85, 

Sulphur, allotropic forms of, 84 

Surface tension, 104, 117 

Tartaric acid, crystals of, (Fig. 15), 

, molecular structure of, (Fig. 

14), 84 
Temperature, and molecular energy, 


and radiation, 47 

radiation, 150-152 

, and quantum theory, 187 

Temperatures, low, and specific 
heats, 119, 120 

Thomson, Sir J. J., 52 

, and meta-neon, 75 
, discovery of the electron, 

, theory of atom structure, 

Thermodynamics, second law of, 

Thermo-electric circuit, (Fig. 21), 


Thermo-electricity, 133-136 
Thermo-electric series of metals, 


Thermopile, principle of, 133-134 
Transparency, theory of, 30 
Traube, J., on atomic volumes, 67 
Triads, of elements, 68 

Ultra-microscope, 58 
Ultra-violet light, 156 
Uranium, life of, 167 

Vacuum tubes, 98 
Valency, chemical, 141-142 
, in periodic table, 69 

electrons, 89, 177 

Van der Waals, gas formula of, 57, 

Vapor, above liquid, temperature of, 

, molecules of, at liquid surface, 

(Fig. 9), 15 

pressure, 775-777 

, laws of, 116 

Vaporization, latent heat of, 104 
Viscosity, of gases, 57 
Voltage, 26, 30 


Water, decomposition of, 7, 66, 90 
Waves, electro-magnetic, gamut of, 

50, 755-757 

, Hertzian, 29, 48, 156 
Weights, atomic. (See Atomic 

Wien's law, 47 
Wilson, C. T. R., photography of 

ionization paths, 126, 169 



X rays, 184, 185, 186 

and atomic numbers, 180 

and crystal structure, 113 

, characteristic, 189-190 

, corpuscular properties of, 


, diffraction of, 191-193 

, discovery of, 53 

, measurement of, 189-193 

, nature of 49, 189-190 

, penetrating power of, 190 

, reflection of, 49, 19 1-193 

X rays, similarity of gamma rays, 

, spectra, source of, in atom. 


spectrometer, 190 
, wave-length of, 156 

Zeeman effect, 29, 34, 149-150, 155, 

(Fig. 24), 150 
, Paul, 149 
Zero, absolute, 20, 107 







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